Is it good looks, musical ability, how they get along with the other band members, or songwriting, for example? Taking 2 bands for arguments sake, but you can take others as you wish - Pink Floyd and The Rolling Stones. David Gilmour, I would consider a real member of Pink Floyd because of his songwriting and his uniquely brilliant guitar work. Jon Carin (who is getting a lot of attention in other threads), I would consider a session player. Although he has contributed some songwriting, I would say it is peripheral to Floyd history. Guy Pratt, who I admire a lot for his musicianship and enthusiasm, I consider somewhere in between session musician and full band member, as he has contributed very little to songwriting. For the Stones, I still can’t consider Ronnie Wood a real member of the band for similar reasons to Guy Pratt. He is without doubt a popular personality in the band and his guitar playing is excellent, but…… is he really a Rolling Stone?