The question remains rather unclear about the setting, so let's look at existing drivers and retarders of migration and see which might apply.

Given that we're not forcing people to travel to Mars, we just need to make sure that the idea (note, not the reality) of living on Mars is more attractive than the idea of staying put on Earth.

The factors which make people want to move generally break down into two categories:

Push factors (things that make you want to leave home)

The two most common push factors we see today are economic (a lack of work) and danger - these both come in many forms, and are particularly strong drivers because they often make living in the source country impossible.

Other push factors are things like lack of freedoms (for example, wanting the freedom to practice their religion or things which don't pose a direct threat to life, but make life in the source country intolerable).

Pull factors (things that make you want to arrive somewhere else)

These are often economic too, but unlike the known realities of life in the source country, are often based at least partly on promises and hopes. People will move to a city because they believe life will be better there, regardless of whether or not that is true (a famous example of this being English migration to London).

Other pull factors can be climatic (think people retiring to the Carribean), idealogical (most famously Cold War defectors), lifestyle based, or any other reason people would want to live somewhere else.

These drivers are then set against the factors that make people want to stay:

Factors that make you want to stay where you are

Familiarity and family/friends are the big ones here. It's a big deal leaving everyone you've ever known behind, and that's doubly true when moving to a different planet entirely. In general, anything which makes you happy with your life in the source country will make you less likely to want to move.

Factors that make you not want to go to the destination country

For frontier countries, this normally means danger, but other things like hardship (or indeed any of the previously mentioned factors) also apply.

Difficulties and costs of migration

If it's difficult, costly or dangerous to actually make the journey, fewer people will make it. This has been true right the way through history, and is still true in the present day (think of the migrants and refugees attempting to cross the Mediterranean - many of them want to make it Europe, but lack the ability to get there).

Applicable factors for Mars

As with all other migration, the biggest drivers will be opportunity (on Mars) and danger (at home). If you can ensure that enough Mars colonists do well for themselves then people will want to go there, and it can be pretty safely assumed that there will still be refugees on Earth who would jump at the chance to go literally anywhere else.

This isn't as simple as it sounds though - in order to get any real numbers of people you'll need to ensure that people can live on Mars, which really means that you'll need to have built (or organised) your colony before people will move in - wealth means nothing if you can't spend it, and opportunities mean nothing if you're not alive to make use of them.

One unusual driver is the excitement and adventure of living on Mars - Mars One received more than 200,000 applicants, even with no possibility of return.

Of course, being on a different planet is a formidable challenge for a colony. Given that regular people don't have the resources to get to Mars on their own, a prerequisite for migration will be an already existing space transport service to get them there. Basic economics tells us that demand and price are inversely correlated, so for maximum numbers of migrants it should be as cheap as possible - financing options such as working one's passage will also help.

Space travel is also risky, so you'd want to make your service as safe as possible too - again easier said than done.

Conclusion

The reality is that, as long as you make Mars colonisation possible, motivating people to move there won't be a problem.

If you build it, they will come

As long as the frontier is a) accessible and b) has some kind of opportunity, people will find a way. Indeed, the question isn't likely how to attract a million immigrants, assuming you can physically transport, clothe, feed, and otherwise provide for them all: it will be stopping at a million.

So think about why people immigrate in real life: jobs, primarily. What kinds of jobs would be available on Mars? Well, it has potential for a lot of different types of scientific research. Once the scientists are there, they need support: they need food to eat, materials to build their homes and workplaces, electricity to power them. They'll need entertainers and street cleaners and architects and lawyers and, in general, all the trappings of civilization. (And all of those people will need food, homes, etc.) Sure, some of these services can be provided telepresently from Earth, but there will always be someone who thinks they can undercut the competition by going local. The market will provide.

If you're interested in hard numbers, you might look at cities in the million-people region and look at their census and demographic data. Try to suss out how many are in primary businesses like industry and research, and how many are supporting them, whether in the service sector, in government, agriculture, etc. You only need to provide impetus for the first part; the second part will come naturally to see to the colony's growing needs.

My answer? Taxation. Think about it- offshore banking, and use of tax havens, is big business. But governments across the world have been cracking down on offshore bank accounts and tax havens, since the turn of the century; increasingly so, in the last few years, and the next few years to come. Now, given the costs of moving to Mars, and the costs of living on the Martian colony in the initial phase, in order for Elon Musk and Space-X to turn a profit, they'll need to make a lot of money per passenger; only rich people are going to be able to afford it anyway. So why not market it, and turn a profit on the colony, by establishing it as the very first offworld tax haven, and offworld financial center?

Market it to all of the world's biggest companies- Apple, Amazon, Alphabet, Microsoft, Facebook, Alibaba, Berkshire Hathaway, Tencent Holdings, JPMorgan Chase, Exxon Mobil and all the rest- as the next step in tax avoidance and tax sheltering. Get them all to shift their main corporate headquarters there, offering them corporate tax rates lower than those of any jurisdictions on Earth, and they'll do it in a heartbeat; in doing so, you'd easily have a chance of making your Martian colony profitable.

With all of the world's largest corporate headquarters shifted to Mars, so do all of the world's most lucrative job opportunities in the corporate sector. Along with those of the banking sector; minimal to non-existent taxation on personal income, wealth and inheritance would be more than enough to induce the world's mega-rich to relocate there as well. Essentially, the Martian colony (/colonies) would be employing a similar economic model as Bermuda, with import duties, payroll taxes and consumption taxes in lieu of income taxes, inheritance taxes and corporate taxes.

The world's governments wouldn't be too happy about it, sure; but what're they gonna do? Space-X now has offshore floating launch platforms, from which its reuseable rockets can both launch and make controlled landings. And this isn't a government project; this is an entirely private effort. With Musk having been the very first to shift his own corporate headquarters to his Mars colony, they couldn't hold him to account for tax avoidance any more than any of the other companies. What are they going to do? Build up a militarized space force, demand the taxes that the worlds' nations are due, and start an interplanetary war over it...?

If we can make it accessible and there are opportunities for people it won't be a problem. If enough infrastructure is in place to deliver building materials for all sorts of development people will take the risk.

If construction materials are can be produced locally then prices will be more reasonable and you can bet that someone would quickly build a resort.

Check out Isaac Arthur outward bound series on youtube for more inspiration on these subjects, highly recommended. Especially one of the later namely the Ceres episode gives a plausible scenario where we are not speaking of just extremely well educated people moving to take the opportunity.

I'm pretty sure you could get at least a million, probably a lot more, volunteers willing to sell everything and move to Mars, I would if I was unattached. I'm only thinking of professionals you'd actually need for the job, there'd be far more who you wouldn't need or want. The only motive many many people will need to take this on, even knowing the real risks, is a sense of adventure. The trick isn't getting a million people to go, its thinning the herd so you can justify to the rest only sending a million people.

How realistic would be to call out for a million people to move to... Antartica? You can test out all the answers here by applying them to Antartica.

Ignoring transport and survivability, neither are a very nice place to live.

Mars is less hospitable than Antarctica, and Antarctica has marketly low migration. Only temporarily manned research bases - arguably, comparable to the space station.

The thrill of adventure motivates the extremely adventurous, but there might not be a million qualified candidates.

So... we arrive at your question. The obvious one would be money/jobs obtained from some resource, such as mining or agriculture. But transport costs of almost all minerals make it not worthwhile (if we account for their transport, not of the colonists). Perhaps some extremely high value/low mass resource? Or, for non-earth use, such as satellites, spaceships, spacestations etc.

Agriculture and spices etc seem to be out.

Tourism could be an industry, but there aren't enough super-rich people to support an industry.

However... there may be something valuable there that we don't know about, and we just wouldn't have thought of before we saw it. That's the true nature of exploration.

Assuming that you've solved the cost and habitation issues (for instance, it's being fully funded by an insane wealthy person), then look at it this way:

Currently, there are some 7 billion people on Earth. That's 7,000 million. You want to move 1 million people to Mars, or less than 0.1% (less than 1 in 1000).

If you're just looking to fill a quota without much care for who goes, it should be fairly straightforward to get to 1 million volunteers. Humans are an adventurous species, and you need less than 1 in 1000 people to be interested. The tricky part won't be finding enough volunteers - you might have trouble finding enough good volunteers, but just filling headcount seems quite workable.

Under current circumstances, a large-scale Mars colonisation driven entirely by economic interests is not going to happen.

• Life on Mars is far more dangerous than on Earth. Deadly atmosphere, radiation, no way to obtain food or water without using lots of technology, etc.


• There will be a very low quality of life on Mars. You are going to live your life locked into a pressurized shelter.


• Space travel is prohibitively expensive.
  
  
  • Those few billionaires who could afford a ticket to Mars have important jobs and enjoy a high quality lifestyle on Earth. They wouldn't want to relocate to Mars, nor would anyone want them to.
  
  
  • Minorities who want to flee from Earth to escape persecution won't have the money to afford it. A refugee camp is not a good place to build a rocket.
  
  
  • Turning Mars into a penal colony would not make much sense because just locking people up in max security prisons for life is still cheaper
  
  


• There are no known resources on Mars which couldn't be obtained much cheaper on Earth or through asteroid mining.

How could you as a world builder fix these issues?

• Invent new technology which drastically improves the survivability and quality of life for Mars colonists and which isn't prohibitively expensive.


• Invent a new space travel technology which reduces the cost for transporting people and resources between Earth and Mars by several orders of magnitude.


• Make up some resource with very useful economic applications which only exists on Mars in order to make a "gold rush" scenario plausible.


• Make the situation on Earth so much worse that Mars seems better by comparison. Resource exhaustion, ecosystem collapse, natural (or man-made) disasters, etc.


• Create a quirky legal loophole which creates a financial incentive for colonizing Mars
  

I can't imagine anyone would be FORCED to go to Mars by any government. While it is true that not having all our eggs in one basket is better for human survival as a whole, no government is currently support that idea and backing Elon. This is a private effort, and Elon has no power to force people to go into space like a government might.

With that in mind, Elon's attempt to get humans onto Mars would be a fully privatized effort and therefore a volunteer basis. If Elon can get his wanted 1 million or falls short by tens, hundreds, or thousands of people is entirely out of his hand. Maybe no one wants to go to Mars and then no one goes.

If a government got behind Elon, perhaps it could be a military style conscription, but there's likely going to be strict physical requirements for the first humans to colonize Mars to ensure they're healthy and up for the task. This also isn't even taking into account mental state for someone who will be mostly cut off from contact with the rest of the world.

You don't need to motivate people to go to Mars: A lot of people will volunteer to go as soon as you need it. Just look at the Mars One project:

• It offered a one-way journey to Mars.


• The viability of the project is not clear, a lot of people think it is delusional or just a scam.

And more than 200.000 people volunteered! Just offer a real opportunity to go to Mars and you will have millions of people eager to go.

Surely with lower gravity, the best reason to go to Mars would be to create infrastructure for further onward travel? If you could mine enough metal it would be a lot cheaper to get that into space. Build your orbital construction platforms for missions to explore and mine the asteroid belt and outer planets.

You don't want the general public going - not at first at least. You want skilled designers, engineers and construction & maintenance workers. Once that population gets big enough, and there is a viable working colony, those workers may want to bring their families over, kickstarting a colony.

Your biggest problem though is going to be food and water. Importing from earth is going to be financially crippling and growing your own is going to have all sorts of risks. Failed crops will not be an option.