Multiplayer: IE Campaign. Joining campaign in progress - does it work?

We are 3 guys who started an IE campaign and we've played for 1 evening.

But at the end of the night a 4th buddy wanted to join us. But when he tried to joined it seemed he took over one of the other guys factions and was locked to that faction?

We didnt test it extensively. And the one he took over had gone to bed.

But is it possible for an additional player to join a campaign that's in progress and take over an NPC faction?

Or do we have to start all over again?