I've provided some custom CSS for this before:
[REQUEST] NodeBB Group Leaders
Hey NodeBB Community,
Looking around i thought about seeing Group Leaders who can manage who can join a group.
Person A = Group Leader
Person B = Wants to join this group
Person A = Gets Notification of Joining Request
Person A = Accepts/Declines
Person B = Put into Group / Not Allowed
Only and idea but there it is.
I would personally love this.
I feel like it will become more feasible once theres some work on enhancing member permissions completed, which I believe to have been roadmapped for 0.5 1.
I'd also love to see this as an option for larger communities that require it.