You can certainly ask Whom are you hanging out with?— it's completely grammatical— though the kind of person who would say it would probably use the even more stilted With whom are you hanging out?
The difference between who and whom has been covered extensively in previous questions. Whom has been suffering a steady decline (in conversational English) for some decades, and sounds formal or affected. As such, you'd be unlikely to hear it used with the colloquial hanging out, as whom is largely absent from less formal registers these days. Additionally, it is familiar to see who at the head of a sentence as an interrogative pronoun, so it is either less noticed by or less objectionable to pedants.