There is no "server" of which I am aware. The player who is loading the save game, or the "host," is the "server" as far as I know. What exactly is happening? It could be a networking issue on someone's machine/network who is trying to host/join. I haven't played MP in a while but I remember the first time being a little problematic. Worked great after we all figured it out. Can't remember if we ended up using Steam's invites or the game's. I do remember problems with distributing the invite code or whatever it's called.