I dont think there is a lobby as such, but you should be able to join a game from the main menu by going:
online-> new match-> selecting 3 player slots and 3 empty slots-> starting player:random-> hitting play-> play now
I suspect someone has done this within the last hour as my matchmaking game has updated its timestamp.
There was also a 4 player game that now has 3 players in it, but I doubt the other 2 are active.