At the start of the manga, Luffy claimed that he wanted at least ten people to join his crew before entering the Grand Line (not counting himself).
Although most adventures in One Piece are random, there seems to be a recruitment “process”:
- First, a situation arises where a new person is needed. The crew meets someone who meets the criteria (e.g. Chopper the doctor, Nami the navigator, Franky the shipwright, Sanji the cook, etc.). Luffy offers this person a job and usually gets an initial rejection. (which usually causes Luffy to comically ‘deny that refusal’).
- Meanwhile, a situation arises where somehow Luffy ends up going up against a “boss” character, and the future teammate goes up against an underling. (e.g. Luffy versus Don Krieg after Sanji confronted Gin and Pearl). The would-be teammate usually defeats one of the villain’s minions or plays a crucial role in the Straw Hats’ victory (e.g. Brook defeats Tararan and reveals the Zombies’ weakness, despite losing to Ryuma).
- During the arc in which the new crew member is introduced, his story and his dreams are revealed (although Nami and Robin’s stories were revealed later), which makes him convinces him to join the Straw Hat crew.
- Meanwhile, Luffy’s current crew helps him fight various enemies, usually based on their abilities and overall strength., which is matched by the person they are fighting (e.g. Zoro vs. Kaku, Sanofi vs. Kaku). Zoro vs. Kaku, Sanji vs. Jabra, Chopper vs. Kumadori, Nami vs. Kalifa), the enemies fought also have similar fighting styles (Zoro vs. Hachi who are both swordsmen, Sanji vs. Kuroobi who are both sword fighting experts. melee, and Usopp versus Chew who are both ranged weapon experts), this is more consistent when it comes to Sanji and Zoro.
Hence the fact that there is a chance that Vivi will join the crew.
The only crew members who didn’t go through this process are Jinbe and the female members, Nami and Robin, who never helped fight an enemy while joining and whose stories were later revealed, though. they were originally suggested. Nami’s official recruitment took place when Luffy, Zoro, Usopp and Sanji defeated Arlong and his pirates. Robin was the first to volunteerand officially joined the group after Luffy’s crew rescued him at CP9. About Jinbe Recruitment, Luffy invited him to join the Straw Hats, but Jinbe held back until he cut ties with Big Mom. He will do this later during the Whole Cake Island Arc and officially become a Straw Hat in Wano.
After the Straw Hats arrive on the Grand Line, everyone Luffy recruits are misfits. (Chopper the reindeer, Franky the cyborg, Brook the skeleton,…)