It sounds like you're saying that the user has a membership relation with A, and B has a composition relation with A. Is that correct? In such case, B is like a sub-grouping of A, and it doesn't necessarily make sense to think of A as being the parent of B. A party that has special privleges over a group, should probably be represnted as a sub-component of a larger group.
Perhaps, I'm missing what you're after.