I joined my friend just after creating my character. He was level 5 so he’d done the first town and dungeon.
I was teleported to him, we found the hermit and ran off to the town and then played through the prologue.
However, since I hadn’t done it on that character, nothing was saved when I logged out and back in, so I had to start over. He joined me again to do the dungeon once more, but after the dungeon, the quest bugged out since I had already talked to Lorath in the city before.
There should be a cleanup of the quest interactions between characters so this doesn’t bug out on launch.
Dailies and side quests were up to date and still completed, but I couldnt for example finish the upgrade potions quest since I hadn’t finished my prologue.
If you finish the prologue on one character, it shouldn’t be mandatory to finish it completely if you’ve done so much of the questline on a new char - it should at least not reset and bug out.
Alternatively, don’t let people party unless the prologue is finished.