Steamgroup:
http://steamcommunity.com/groups/HGN-Left4Dead (http://steamcommunity.com/groups/HGN-Left4Dead)
For other groups edit the end of the url to HGN-(Game name)-(Game mode if multiple servers for that game)
I joined, lets play a game when a bunch of us are on.
You should have officer, make an announcement whenever you wanna play.