Idea: Can view/Can comment/Can edit/Ghost PMs:
Jack, Jill, John, and James. have a PM.
Jack starts the PM. He doesn't want Jill to know John can view the PM, so he sets John to 'Ghost'.
Jack wants James to only be able to comment on some ideas, so he sets him to 'can comment'. Comments would work like replies that appear in margins within hider-like things.