Welcome to your interview.
The question is on your right: Aggregate Permissions in Group Graph. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Design an object-oriented model to represent user groups with nested parent-child relationships, and write functions to aggregate permissions across these groups.