All I know is there is an application process, and a limited number of spots. I am assuming there is some kind of admissions vetting process with the leadership in USAG and USOC/OTC.
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Seems to me like the residents there are a mix of post-college past olympians who haven't married and settled down yet (Sam, Chris..as opposed to Jake and Alex Naddour who have wives and lives...), post-college up and comers (Eddie, Donothan, Kanji...) , and some who forego college to train after high school instead of NCAA (Donnell, Cameron Bock, Marvin Kimble (formerly OTC now back to his home gym)..)...
But the exact criteria is a black box.