The invite only thing doesn't really mean anything in practice, though. I think it was more hype generation from Valve than anything else and if you check the Steam discussion page for the game it's just filled with endless threads of people giving away invites. Currently Deadlock has a 96,000 24 hour peak on Steam, putting it in the top 15 games being played on the platform and its all time peak is 171,000 from a month ago. There are definitely enough people in the pool for balancing to be possible.