OK, so there are no game mechanics, but because this still isn't reality we do have a few conventions to simulate things.
First, there is no violence of any kind allowed in the game. You cannot use physical means to make a character do anything or to prevent them from doing something. Locked doors may not be forced. As in real life, the characters should all assume that any act of violence during a party will be easily discovered and severely punished. (It's amazing how many live games seem to exist in worlds where this is not the case.) For purposes of characterization, this means that you should avoid playing characters who might ignore such negative consequences; Mr. Cavanaugh did not invite any sociopaths to his birthday! Player characters may, of course, use the threat of violence after the party in order to influence others.
Second, any signed piece of paper which a player can produce at the end of the game is considered a legally binding contract. For the sake of convenience, we'll assume that all characters have the requisite legal knowledge to spot and avoid an unenforceable contract.
Finally, standard house rules apply. Don't dig through drawers and cabinets, but also don't hide things there. Respect the property and privacy of the real owner of the premises. One special caveat: the standard rule about bathrooms being off limits to game play does not apply to this game. In keeping with the "as real as we can make it" premise, players should remain in character at all times. So if you want privacy, lock the door!