Need have it that what ever time is left in the dungeons you got left then unable to join back into the duty finder untill your that time runs out.
Then have a vote system in place if 75% vote leave or more in the dungeon and they all vote to leave then you can leave the dungeon like online football game forfeits. No issue
Pick up Groups are PUGs. They can bee good or terrible. One way to stop the terrible ones.