So how does the multiplayer campaign work?  As in, if I join someone else's game, am I advancing my guy at all?  And if someone else jumps into my game and stars collecting stuff/doing the quests, does that stop me doing them? 
 
Apologies, I don't have the manual for the game as I've rented it.

It keeps track of orbs but not missions. You still keep all the experience you gain. Oh, and people doing objectives in your game does complete them for you so if you want to do them yourself don't do that. But orbs are separate for each person.

You advance your guy regardless of whose game you are in, as long as it's on your save and if someone does join your game they can start doing your missions regardless if you are near them or not. I set to play the game alone, then I can invite people on my own accord, I like my friends and all but I can't stand it when they join my game without asking.