It's illegal, so they can't sign that as a contract, but they they can just agree to do it.

因为是违法的,合同也就无效了,不过他们可以私下达成默契

耶鲁公开课 - 博弈论课程节选

Why do you manage to get some cooperation between you and your roommates even without a written contract?

为什么即便没有书面合同,你一样可以和你室友合作得愉快

耶鲁公开课 - 博弈论课程节选