K, let me speak siku's words. You just proved to be an idiot.
Alien Swarm is a Valve game, it costed much more than 98% of indie games including Toribash. You seriously don't think that Toribash has any chances of looking like it, do you?

Regarding steamworks, I'm nod a developer actually, so you'd better ask them.
But invitation system and friendslist could be easily integrated, that's what I'm almost sure.
Different accounts? Don't tell me you never heard of connecting different ones. Users with older accounts will be able to continue using their old ones which will be connected to their Steam profiles and newer players will create those which are based on their Steam account by default. That's just one way of implementation.