No, it's perfectly possible. I prolly the easiest and most efficient way of accomplishing this would be to write a listserver program, and when it gets sent a packet that said someone joined/left, it would go and check the server and see who it was that joined/left. Then someone would host a listserver with that, and everyone else would run a client program that would connect to that listserver and retrieve who's currently online.

Well, actually, the easiest way would be to tell your friends to get on an IM of some sort and put up and away message when they play jj2.

-cheeze