From Kevin Connolly, BBC News.
Membership of the Madoff fund was very strictly by invitation only – merely being rich was not enough in itself. […] The very respectability of the clientele helped with further recruitment.
Your community will attract more of the members you have.
If you want keen photographers to join your community, ask as many keen photographers as you can find to join. But if you want the best photographers to join, only let the best photographers join.
Your membership list matters more than your website.