Time commitment:
Minimum of 1 post every 24 hours to avoid being modkilled(bar weekend, where there is no modkill timer), though it's better to contribute a bit more, makes it more fun, and people don't get pissy.
Not sure what you mean by how it goes down on the forum?
To play you are given an alternate account and it's password, there is a game thread(currently stickied and locked in the mafia section) and the game takes place in there, your role will be pm'ed to your alt account.
Hope that helps, and you're welcome to play