What is it?Edit
Artemus claimed they were watching him to determine if he was worthy of them. This tells us that membership is invitation only.
Who are members?Edit
We don't know. No wizard in the show was ever actually stated to be a member.
What other evidence is there?Edit
In Challenge of the Wizards, Sholow met with Chom, Mazem, and Skorpos prior to the episode to determine the terms of the wager. How? Also, who was going to enforce the terms of the wager? What was stopping them from simply killing Sholow for the helmet?