I've decided to organize a friendly tournament with some few friends of mine and their girlfriends. We usually play T1.
What are the rules for such a play ?
if it's just plain old free for all, there's not much rules other than the standard one.
You must decide how you will deal with priority issues. Normally, it's active player, then non-active player. In multiplayer, you habitually go from active player and going clockwise with the priority.
Also, many people have different rules considering combat. You should decide first if you can split attackers between multiple players or no.
Other than that, the game is pretty straightforward, with most cards being worded for multiplayer play as well as 1 on 1.