I think it is just vernacular... if people associate with them long enough the media and everyone else is likely going to refer to them as "members" of the Outfit.
The only formal desgination I've ever heard of is "made."
It is somewhat confusing; for instance yesterday Ivy and I were discussing if Harry Aleman was in fact "made." Maybe he wasn't, as his dad was Mexican. But he was pretty much the archtypical and one of the most notorious Chicago gangsters of his era. Like I said, some people refer to Marco D'Amico as not being "made." Why he wouldn't be, I have no idea, he is by all accounts about as influential and involved in the Outfit as one could be.
