As has already been answered and should be common knowledge, you can join servers using
steam://connect/ip:port
or as viewed here. However, I would want to know if the same is possible for creating a Mann versus Machine Lobby, especially Mann Up and filling it with people by inviting them or giving them a link to a specific lobby. Is this possible?
Best Answer
Clearly the Steam protocol supports joining lobbies when opening a game, but the documentation for it doesn't list what argument it uses.
Then again, you can't join lobbies without an invitation and you can't invite people who aren't currently on Steam... which would make this not useful even if we did know.