My save game now is essentially corrupted after I had a few friends jump in to my solo playthrough. I cannot remove them from my party at all, so I can now never bring NPC's with me on my adventures.
This seems like an oversight, and the only major issue I've had with the game so far. I've already got a muliplayer game with other friends and don't want to have to start a new game.
Everything else has been amazing so far!