You would benefit from setting a fixed number of people they will allow to join the group. You can be picky about members and accept the ones with real influence, skills, resources or commitment to offer.
You can draw up a list of potential members and only invite the best of the best. These are most likely to join because it’s exclusive (and they feel appreciated for being the best).
This week I’m with UNHCR in Davos (a tiny Swiss town in the mountains). 2,500 of the world’s most influential people are here to solve the world’s most important problems. Davos was specifically chosen because it was small and private. It’s invite only and hotel space is limited (exhausted).
If you want Eric Schmidt, Martin Sorrel, Nicholas Sarkozy, Ban Ki-Moon and the most elite people in your field to join, you can’t create a community for just anyone you have to create a community just for them.
I’m betting right now you can create a community for everyone to join or you can create a community for just the elite people to join.