Sup Dude,
I don't know if P2P is really what you want for this case (it wouldn't support, say, offline messages or even get the order of messages right) - it's pretty easy to do this w/ AFCS. Just set up a SimpleChatModel (it's in the SharedModel package), and when you send a chatMessageDescriptor through it, specify a recipient (the userID of the specific person you're talking to). This way, everyone in the room can be talking through the same model, but each conversation is kept private.
hope that helps
nigel