The requirement is anyone over 1800 rating in Diamond league to join, although this requirement may change as ratings fluctuate. BoxeR did note that he is not going to take in anyone who already belongs to a clan or a team.
Edit: Tanatos linked a PlayXP article, and just as I suspected, the netizens wanted proof at first. Then it was revealed on Gisado that this is true. So it is not a rumor.