When it's not for profit, you could agree to share your work under the terms of an open source license (Yes, Mr. Stallman, I know that the term open source is missing the point). Should you ever decide to split up, each of you will have the rights to the full project and can decide to continue development independently, with or without help from additional people.
When you want to make sure that none of you ever monetizes the game without permission of the others, you could use CC-BY-NC. That way you all can can use it, just not for making money.
When you want to allow each other to try to make money with it independently but you don't want them to obtain exclusive copyright, you could use the GNU General Public License. This, however, will also give end-users the right to use and modify your work.
Another option would be the BSD license. It doesn't force you to give rights to the end users. But it also doesn't prevent anyone of you to fork the project and keep exclusive copyright to the modifications he does after leaving you.
Using a free software license will also make it easier for you to find contributors who don't have financial interests. Many hobbyists (me included) would rather contribute to a project under a stock open source license than one where the copyright situation is unclear and you risk getting exploited for profit.
Or you can just roll your own license agreement. Just make sure that you agree how you want to handle copyright when one of you wants to continue without each other due to creative differences and how to handle any profits and losses. Just a hint: Do not agree on splitting profits evenly. You will soon realize that you all invest different amounts of time and energy into the project. For those of you who feel that they did more than the others, an even split will soon feel very unfair.