I just bought the game. When I try to join a lobby it says "joining" for a split second then goes away and nothing happens. Restarting doesn't help.
So, turns out that there are some games that are not joinable for some people. We are trying to figure out why this is happening, and for whom.
With that said, there are also some cases where it can happen if you have your windows firewall turned OFF, or your antivirus has quarantined game files or that you are using IPv6.