It'll be a hard call
Why? Let's say the limit was set at a post count of 1,000 - That's some 250 odd members, since most of them are posting enough that someone from say the first 15-20 people added will know them they can be added one by one via a vouching system instead with a warning of some sort keeping the group private.
It's similar to the concept of being 6 steps away from the president ...