Consider changing the number of Memberships you are allowed to have.
Consider having the number of memberships allowed tied to your 'Level' or 'Tier'
Example: I am a member of a Club (co-workers) and have another set of friends who have invited me to their Club...... (but I can't join the second club, since I am only allowed to be a "member at one club.")
I feel once a player reaches a certain Level / Tier, they are taking the game a little more serious than someone who just signs up and quits after the first week or so.
Thanks