There seems to be an odd pattern with the female Strawhats. Luffy actively sought out or invited Zoro, Ussop, Sanji and Chopper. But Nami and Robin both pretty much invited themselves, that is to say; at first Nami thought she could get good protection/treasure if she stuck with Luffy and Zorro. She was there through a major ark, that is the ark where Luffy gets Ussop and Merry and into the Sanji/Don Krieg(Kreek?) ark. At which point she realizes she has other problems (Arlong) she has to take care of, realizes she has come to consider Luffy and others friends and decides that she has to go away and make the others think she hates them in order to save them. Luffy and others then pursue her, Ussop gets the ever lovin' crap beaten out of him by Nami.....in order to save him from Arlong. Then all hope seems to be lost, Nami starts to break down and in one of the best scenes of all One Piece: asks Luffy to help her, and Luffy and friends do help and Nami becomes the official navigator of the Strawhats.
Nico is interested in the "D." aspect of Luffy, helps him in Alabasta, and appears on Merry claiming Luffy has to take responsibility for his saving her after he beat Crocodile. Robin, at first, is interested in Luffy and sees the Strawhats as good protection/chance for information. She stays through the SkyPiea ark and finds that the Strawhats are true companions. Into the Water 7 ark her past comes to haunt her and, to save her friends, she has to convince everybody that she can't/dosn't want to stay with them. Everybody pursues her and, once again, Ussop/SogeKing is given some "tough love", in the form of a Clutch by Robin, in order to save him and every body from the Buster Call. And when everyone finaly catches up to Robin, she breaks down to their persistance and asks Luffy to help. That brings us up to the current ark.
Now I don't think Vivi counts, as from the start; the point of her being there was to save Alabasta, that is say: did she actually have an official role as a Strawhat?
I don't think there is anything wrong with the aspect of Nami and Nico both being "hard to get", but I do find it odd that the only other Strawhat to have such trouble with Luffy in the group is Ussop.