Hmm...I think you may be a little confused. If an organization does let graduate students join collegiate chapters, but offers AI (and not all do), it is probably handled through an alumnae chapter, not collegiate. In order to "join" an alumnae chapter you need to be initiated, whether that's through collegiate or alumnae initiation.
I don't know if that helps at all of if I've just confused you more
Can anyone else explain it more clearly?