Collins was already in the band (IIRC he may even have been a founding member). The difference is he wasn't yet the frontman – he was the drummer. Pre-Collins-fronted Genesis is way better though, IMO (I do like me some progressive rock, though).
The things I learn visiting here amaze me. I liked their music but never placed a group/faces to songs I saw one on MTV with PC as lead. I thought they formed the group with that format.