Greetings, TI community!
The Mann vs Machine servers we run (scroll to the bottom of any teaminterrobang.com webpage to see which servers are currently running MVM) all use the password: "apollo".
Sometimes it seems to be broken to try to join a server that has already begun the first wave, but other times it seems to be possible to join a game-in-progress via console, using two steps:
... where "X" is, of course, the server number.
We highly encourage players to meet at http://chat.teaminterrobang.com and plan a group before just randomly joining a server.