when does chopper join the crew

Chopper joins the Straw Hat Pirates crew as their doctor in One Piece in Episode 91 of the anime and Chapter 153 of the manga. The story of his joining unfolds on Drum Island, where he initially is wary of humans but eventually accepts Luffy's invitation to join the crew despite his doubts about being a monster. He officially becomes a crew member during the Drum Island arc around that episode and chapter.