Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Making an NPC follow the party?

Also have a look at http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=629934, it seems someone else has already figured this out (and by using status effects, it will indeed be persistent across saves even in GM mode).