Looking at the rules for Sushi Go Party and the zero sum clause is kind of hilarious here. The concept of the game is that you're all trying to eat some really good sushi. But if your sushi meal isn't the best then you Lose. You Fail to Win The Evening at a Sushi Restaurant. The deliciousness of your friend's meal diminishes your own.
The thing is, given arbitrary contract enforcement, blackmail becomes impossible, afaict? Functional decision theory is immune to blackmail, supposedly, but I'd like to take a closer look at how, one day.