Group Member Filter for Public Events

I realize that when an event is public, any registered member can make a booking. To allow only group members to make a booking, it has to be made a private event.

However, if all group events were private, the event list would be empty and the site would not attract new members.

I would really like a way to filter bookings so that it checks if a person is a member of that group before it allows a booking.

Obviously the plugin was never designed to do that, but I was wondering if anyone could come up with a filter to do that, or a change to a file somewhere?

That sort of thing is beyond my skill level, but would really be a useful feature.