Posted from my comments in the spoiler thread with new chapter references at bottom:
IMO, the numbering Oda is using is:
1st Person - Zoro
2nd Person - Nami
3rd Person - Ussopp
4th Person - Sanji
5th Person - Vivi
6th Person - Chopper
7th Person - Robin
8th Person - Franky
where the number is for who Luffy met in chronological order.
I've always felt Vivi might rejoin at a later date, especially because of her unique character type/personality that isn't currently in the Strawhat crew (sweet, polite, royalty).
Like others have said, I didn't find Vivi gelled that well with the other Strawhats while she was on the crew, but maybe her time away has given her more confidence and strength (maybe Carue eats a Devil Fruit!?) so that if she rejoins, the interactions with others will be more interesting.
Also, it's kinda interesting that the next One Piece movie from Toei (Movie 8?) is a re-telling of the Vivi arc… .
As for when people actually get the nth Person designation, I think it is when they themselves ask (thus accept) to be part of the Strawhats without any sort of offer from Luffy. Zoro, Nami, Sanji, and now Ussopp, Robin all asked Luffy to join the crew or to help them after initially declining his offer and/or after some realization.
On the other hand, originally, Ussopp took Luffy on his 1st offer to join the crew. Chopper also accepted Luffy's offer (after some running away and stuff), and Franky was the same after being "persuaded". As for Robin, she just appeared in the crew. So I guess you could ask in the cases mentioned above, whether Ussopp, Chopper, and Franky only joined because Luffy asked/persuaded them, or whether they did it of their own choosing (if he didn't repeatedly ask/persuade them, would they have gone to him and asked to join?):
The First Person - Zoro
[Chapter 5]
Luffy: "Resisting the Navy here together with me will make you an outlaw. Or maybe you want to die here?"
Zoro: "Are you the offspring of the devil… Forget it... Rather than to die here... Why don't I just accede to your request... and become a Pirate!"
The Second Person - Nami
[Chapter 81]
Nami: "I told you to get off of this island."
Luffy: "Yes, you told me."
Nami: "Luffy… Help me..."
[Chapter 94]
Nami: "Gen-san. Nojiko. If Bellemere-san was alive, do you think she would stop me from becoming a Pirate?"
The Third Person - Usopp
–------------------------------------
[Chapter 41 - Not really joined]
Usopp: "We have to meet again someday on the sea."
Zoro: "Just shut up and get on this ship."
Usopp: "Eh?"
Luffy: "We're on the same team now!"
Usopp: "Huh? I'm the Captain right?!"
[Chapter 438]
Usopp: "Can you guys accept my apology!? I really want you guys to forgive me…! Please...! Let me be a Strawhat again...! Please take me back as your Nakama!!"
The Fourth Person - Sanji
[Chapter 68]
Luffy (talking to Yosaku, not Sanji): Let's go
Sanji: "Wait. I Will Go Too. Please take me with you. You want to become the greatest Pirate, right? It means we both have a crazy dream. I will do what I want from now on. I will become your chef."
The Fifth Person - Vivi?
–------------------------------------
[Chapter 216 - Not really joined]
Vivi: "I'm here to say goodbye!"
Luffy: "What did you say…?!"
Vivi: "I... can't go with you all! Thank-you for what you've done for me! Even though I still want to go with you, but right now I... I really love this country! So I can't come along!"
Luffy: "Really!?"
Vivi: "Even though I... want to stay here, but if there is one day where we meet each other again, will you all take me as your friend?!"
The Sixth Person - Chopper?
[Chapter 152 - Not really joined]
Luffy: "Hey, you want to be a Pirate with me right!?"
Chopper: "That's impossible."
Luffy: "Why's that impossible; a Pirate's life is really fun!"
Chopper: "I really want to. Thank-you. But I'm a reindeer! I have hooves and horns. I also have a blue nose! Actually, I want to be a pirate too. But I'm not human. I'm a monster! I can't be friends with humans; that's why I came here just to thank-you. Thank-you for asking me."
Luffy: "Just shut up and come with us!!"
Chopper: "….... Ok!!"
The Seventh Person - Robin?
[Chapter 218 - Not really joined]
Robin: "I have no where else to go. So let me stay on this ship."
Luffy: "Oh, if it's like that, then there is nothing I can do. It's ok."
[Chapter 398]
Luffy: "Robin! I haven't heard it from you yet! Say you want to "live"!!"
Robin: "I want to live!!! Take me with you… to the ocean!!!"
The Eight Person - Franky?
[Chapter 397 - Not really joined]
Luffy: "Hurry up Franky! Get on my ship!"
Franky: "… Hu, look at you talking... For a bunch of idiots who only do crappy fixes, this beautiful ship will suffer without a shipwright. That won't do. Oh well! It can't be helped. Then let me look after it! I, Franky, accept your invitation to be your shipwright!!"
Also, an important conversation from Luffy/Zeff in Chapter 68:
Zeff: "Can you take ... Sanji with you?"
Luffy: "No."
Zeff: "What do you mean kid? Didn't you need a chef; what's wrong with Sanji?"
Luffy: "Nothing's wrong with him; I want him to come with me. But he said he wants to stay here. Even though you want me to take him... I just can't."
Zeff: "You mean, you can't take him with you unless he said so."
Luffy: "Yes."
If this theory is right, and a chronological order has to be kept, could this mean that Vivi WILL join as the Fifth person?