For the horse retrieval quest at the Westfall Brigade camp if you just park your slaves where you turn in the horse ALL will get credit!

This is another method I've used as some quest just required the slaves to be there even if they didn't participate.

Another that came to mind was that 'Blood in the Water' quest from Death's Rise where you have to use an object on a corpse. I was like ah man have to kill 50 sharks to cover everyone and almost turned it down. Then I went and tried it and everyone got credit for 1 kill so don't leave a quest before you see what it's all about.