Incredibly annoying bug or "as designed", but whenever my friend joins me in my game, we have to dismiss the companion he wants to control and then we have to go to the hall of heroes and he specifically must ask him to get back into the group.
Why is this and can't you make it so that a save game remembers who the joining player last controlled?
The game just crashed for my friend and when he was able to rejoin my game again, we also had to dismiss his companion and make the journey...