Is there any reasonable way to add someone to a multiplayer party once the game has already begun? I'm not talking about editing or anything like that, any mods or other ways?

Thanks

Im not a 100% sure what you mean, but i think you're asking wheter or not you can join an already started multiplayer game. if so, all you have to do is assign one of the charater slots to the new player, then he has to click the create character button and create his character.

you can have NPCs join and assign them to an appropriate player
you can distribute existing characters between gamers and you can of course delete and add any char you want. All you need to do is to go to the options (button with two PCs on the left).

Sorry I was unclear, but I like to hear you can add people later.