Possible? Maybe. Through some brilliant reverse engineering, people with SourcePawn have managed to convert Steam IDs into Friend IDs, I'm not certain if there is a way to access group IDs, however. Probable though? Of course not. It'd be hell trying to make a plugin to do it if it is possible. And the network overhead to check each player if they're in one of the groups would be painful
Well, Recon's old ESL plugin used to only allow people in a specific steamgroup to join a server. And sourcebans supports steamgroup banning. So theoretically, having it check how many people are in the server from a set list of steamgroups should be possible. But I would honestly say it's not worth the effort for the few maps that need this. Instead, let's just force them on teams that can't play them properly, and maybe they'll learn the hard way. It might suck for a bit, but in the long term, the average player skill should go up, back to how it used to be.
I was actually thinking that, some games do after all do matchmaking systems with people on your friends list or in your groups, so you should be able to do it with steamworks.
