Make your community valuable and your invites scarce. That's a good way to
Many community members, if offered an invite, still wont use it. They will
get another one next month though right?
What can you do about that?
Tell members to use their invites or lose them. Either they use their invite
this month, or they wont get any next month.If they do bring someone into the
community, they get twice as many next month.
Now your inactive members have a fear or loss to contend with. Your active
members have twice as much to gain (and lose).