sanji was not the leader of a group his teacher was the leader everyone just looked up to sanji as well. franky wasn't the captain of a ship they were hooligans and he just leveled up to pirate. thats like saying usopp was the leader of a group because of those kids.
bartolomeo and law actually have pirate crews and are captains neither of them are joining and neither is jimbei because he's an already established name and a former warlord it would make no sense for him to join a rookie team who's coming up and making a name for themselves.
"Luffy isn't opposed to letting
members or leaders of another group join."
Please read more carefully next time.
Pirates are a group of thugs who share a close bond and travel the sea. Franky's group was a group of close thugs who were in one place. The only difference between Franky's crew and a pirate crew was that Franky's crew didn't sail the seas.
The Strawhat Crew has already made a name for themselves. Zoro had made a small name for himself as well before joining Luffy, to the point where a Shichibukai's organization had been considering and scouting him even before he joined Luffy.
The point is, Luffy isn't opposed to letting captains of another crew join, as seen with Franky, Jinbei and Law. The fact that Bartolomeo is a leader of a group is not a good enough reason for why he wouldn't join because Luffy doesn't give a damn as long as he leaves his group on good terms and doesn't just abandon them, and Luffy has final say, not you.
