(this is plugin territory, but I couldn’t select that category)
Can’t create it myself, but just to suggest the idea:
Some communities might work on invitation only. But in a members only community it would be nice to have an invitation only be valid after a certain amount of people have invited that person.
I don’t know what happens if multiple people invite the same mail address. If you track it, you could enforce the above idea. If someone clicks the invitation link before reaching the amount of people needed, he would see how many more he needs before allowed entrance.