ok so when you are doing the act 2 part 8 with 4 people the first person can get the handin 8 times for one boss kill heres how itll work
1) 4 people join
2) kill the boss in soul chamber
3) 1 person needs to leave before you do the cutscene and get the stone he will be the last person
4) now everyone else leaves and resumes their own games except for the first player
5) now after he hands the quest in to the first 2 other games join the last persons game
6) since he left before you got the stone the objective should be get the black soulstone
7) get the stone and everyone leaves and resumes their game again letting the first guy get the quest xp 4 more times
you can actually have more then one person not getting the stone but i think that would get to hectic