Yo
I was thinking a little bit about this and here is my analysis:
-1st crew member = Monkey D Luffy
he s the captain because he wanted and began the crew
his goal is to be the kaizoku ou and for that he needs to find "One Piece"
he planned to do that with ten crewmembers (or captain + 10= 11members or captain+9=10m)
flashback = first chapter so we saw Little Luffy, and why he wanted to become kaizoku ou, before we saw grown up Luffy
-2nd Crw m = Roronoah Zoro
he s the first mate and decided to join Luffy because he felt that he could realize his dream with him
his goal is to become an invincible swordsman and for that he needs to win against Mihawk
flashback = before he decided to join so we saw little Zoro, and why he had his dream, before he became a Mugiwara member
PS: Luffy heard about Zoro and decided that he needed him in his crw to become Kaizoku ou
Zoro acknowledges Luffy as captain since the beginning.
-3rd crw m = Nami
she s the navigator and decided to join Luffy because she felt that she could realize her dream with him
her goal is to draw the map of the whole world and for that she needs to travel around the world
flashback = (I think that Nami really joined the crew after the Arlong-arc, because she said the first time–> Nami:"I m just cooperating with you while it serves my purpose remember that" Volume 3 chapter 21 page 2 and she never called Luffy captain before the Arlong-arc, so:flashback=)BEFORE she decided to join (I think she decided to join at the grave of Belmer-san) so we saw little Nami and why she had her dream BEFORE she became a Mugiwara member
PS: Luffy bounced on Nami by chance and heard that she was a navigator, after that he wanted her to join
Nami didn’t acknowledge Luffy as captain only after the Arlong-arc
-4rth crw m = Ussop
he s the sharpshooter and joined Luffy because he always wanted to become a pirate and Luffy asked him to join him
his goal is to become a brave warrior of the sea
flashback = After he joined Luffy on his ship so we saw little Ussop AFTER he joined
PS: Luffy bounced on Ussop by chance and wanted him to join
Ussop don t acknowledge Luffy as a captain (when he joined he shouted loud I M THE CAPTAIN and later he said :"okay I give up however, if you don t know to do anything you have to make me captain of this ship" volume 5 chapter 42 page 2
also in the Water7 conflict between Ussop and Luffy, he said:"don t say I’ve decided like you’re a real captain" and "when you think back to when I joined, I only went because you invited me" volume 35 chapter 331)
-5th crw m = Sanji
he s the cook and decided to join Luffy because he felt he could realize his dream with him
his goal is to find all blue and for that he needs to travel on the Grand Line
flashback = before he decided to join so we saw little Sanji before he became a Mugiwara member
PS: Luffy and his crew were searching for a cook and went for that purpose to the baratie
Sanji acknowledges Luffy as captain since the beginning but he s the only boss in the kitchen
-6th crw m = Tony Tony Chopper
he s the doctor and decided to join Luffy because he always wanted to be a pirate (his mentor learned him that)
his goal is to cure all the diseases in the world
flashback = before he decided to join so we saw his story, and why he had his dreams, before he became a Mugiwara member
PS: Luffy and his crew were searching for a doctor, he asked Crocus (Laboon-arc), he asked Dr. Kureha and then asked Chopper to join(without knowing he was a doctor of course)
I think that Chopper acknowledges Luffy as captain since the beginning
-7th crw m = Nico Robin
she s an archaeologist and decided to join Luffy because she had nowhere to go, later she found out that she could realize her dream with him
her goal is to learn about the lost history and for that she needs to find the rio-poneglyph
flashback = after she joined Luffy on his ship so we saw little Robin, and why she had her dreams, after she joined
PS: Luffy bounced on Robin by chance and when she asked him if she could stay with him, he accepted
Robin acknowledges Luffy as captain since the beginning
-8th crw m = ?????????????
all we know is that they went to Water7 to repair their ship and to find a shipwright as a new crew member
the same happened when they were searching for a cook
-PS: thanks to loulou I noticed something about the chapter titles I ll just quote him:
@loulou:
When Zoro and Sanji join the Strawhats, the titles of the chapters say "the first person" and "the fourth person." When Nami joins the crew again in Arlong Park the chapter is called "the 2nd person." This seems to indicate that those characters are the ones who will be devoted to Luffy no matter what from now on. The others (Usopp, Robin and Chopper) weren't introduced to the crew this way, and Usopp and Robin both left. But when they join the crew for good, the chapters will say "the 3rd person/6th person." Chopper hasn't been introduced as "the 5th person" which suggests that he may have some conflict later too. However, it doesn't necessarily mean Chopper will leave the crew, maybe one incident or moment will assure Chopper that he really belongs with the Strawhats.
Conclusions:
Zoro, Nami, Sanji are 100% mugiwara-crew members
Because: -we saw their flashbacks before they entered the crew
-they acknowledge Luffy as the captain and have total fate and trust in him
-Oda clearly mention in the chapter titles that they are the 1st, 2nd and 4th members
Chopper : the only problem with him is that he wasn t introduced as the 5th member
Maybe because: -he s in fact an animal and not human
-Luffy didn t clearly acknowledge him as the doctor with a great dream
Ussop, Robin: clearly they still aren t 100% mugiwara-crew members
Because: -we saw their flashbacks after they entered the crew
-they still don t have total fate and trust in Luffy and Ussop don t acknowledge him as captain
-they weren t introduced by Oda as the 3rd and the 6th members
PS: Little comment about Vivi:
-we saw her flashback after she joined
-she never saw herself as a pirate so she never acknowledges Luffy as her captain
-she was given the choice to really join the mugiwaras but she didn t accept
à so she s not a mugiwara