Just to be clear on the math, I include both the introductory chapter and actual joining in the count. So if a character were introduced in Chapter 5 and joined in Chapter 10, I’d count it as Chapters 5, 6, 7, 8, 9, and 10, so 6 chapters total. The equation’s not Join-Intro, it’s (Join-Intro)+1.
Luffy: 1 chapter. He was introduced and starts his adventure in Chapter 1, and while I guess you could argue he didn’t “join” a crew until he actually had one, let’s just say the captain is the first member.
Zoro: 4 chapters. He was introduced in Chapter 3 and joined in Chapter 6.
Nami: 14 or 87 chapters, depending on your point of view. She was introduced in Chapter 8, starts traveling with Luffy in Chapter 21, and officially joins after Arlong’s defeat in Chapter 94. Personally I favor the latter number.
Usopp: 19 chapters. Introduced in Chapter 23 joins in Chapter 41.
Sanji: 26 chapters. Introduced in Chapter 43, joins in Chapter 68.
Chopper: 19 chapters. Introduced in Chapter 134, joins in Chapter 154.
Robin: 105 or 285 chapters, depending on your point of view. She’s introduced in Chapter 114, joins the crew after Alabasta in Chapter 218, and resolves to join permanently at Enies Lobby in Chapter 398. Unlike Nami, I prefer to think of her as joining at the lower number.