Either of these works:
You are very good.
You have been recommended by a clan member.
Your a good friend of one of the clan members.
You have been invited.
(Very rarely) your application has been accepted.
Just apply, tell us your name how often your on and who you want to train you. (Duke or Gooberman).
List of permanently banned people from skill:
-Coke
-Redline
-Samir