How can I safely accept volunteer work?

You need to talk with a lawyer who can provide you with a document that enforces that.

There are various types of documents that apply. A collaboration agreement is generally used when people are peers working together. A rights assignment or employment agreement or independent contractor agreement are generally used when the rights are consolidated to a single source (this is typical in the professional world). Incorporating into an LLC or S-Corp requires articles of incorporation and registration with government agencies, that's typical if you are going to run your business in a businesslike manner.

The documents (collaboration agreement, rights assignment, IC agreement, etc) will specify who owns what exactly, and what payment will be made if any.

Without any of those properly written legally binding documents each person will individually own their own contribution. It gets into some nasty situations with who owns specific rights to what aspects of the product.

You could also do an independent contractor/consultant agreement. You can probably find free forms through a quick google search. Basically they are consulting you on your work and they are compensated with a copy of the game. Since it would qualify as a "work-for-hire" anything they contribute is owned by you.

IMPORTANT: There are many useful and quality free legal resources online. When using a free legal resource please remember to balance risk with cost. A free document will save you money but is more likely to have a defect that will cause it to fail. If you have enough invested it is always worth it to pay for the protection provided by professional advice from a bar admitted attorney.

Basically they are consulting you on your work and they are compensated with a copy of the game.

The problem with that is you can fall foul of various minimum wage laws if the time your consultant spends consulting is longer than could adequately be paid for by a copy of the game. So unlike a typical contractor agreement you couldn't specify a minimum amount of hours worked or deliverables to produce.

Basically they are consulting you on your work and they are compensated with a copy of the game.

The problem with that is you can fall foul of various minimum wage laws if the time your consultant spends consulting is longer than could adequately be paid for by a copy of the game. So unlike a typical contractor agreement you couldn't specify a minimum amount of hours worked or deliverable to produce.

Minimum wage laws do not apply to independent contractors. The only thing you have to do is make sure you don't cross the line between contractors and employee.

And that in itself is a minefield that varies from one jurisdiction to the next. Unless you're working exclusively with people in your own country (where a standard form would suffice) I'd say it's better to just steer clear of anything that implies agreed compensation in return for agreed work, if what you really want is volunteers.

And that in itself is a minefield that varies from one jurisdiction to the next. Unless you're working exclusively with people in your own country (where a standard form would suffice) I'd say it's better to just steer clear of anything that implies agreed compensation in return for agreed work, if what you really want is volunteers.

At least in the U.S. its really not that complicated:

"What matters is that the employer has the legal right to control the details of how the services are performed."

https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-defined

So basically as long as OP doesn't overly control (which wouldn't make sense since the OP just wants them to come in and give some opinions) he is fine. Also since I doubt OP will be a slave driver with his volunteers, the risk of any claims is going to be nothing and regardless of the employee/contractor choice the IP is his when all is said and done.