I think luffy will say no we can just always be friends
It's not a confession you know >.>
Realistically there isn't any reason for Luffy to refuse whatsoever. They're not asking to become Strawhats, they're asking to become subordinate crews and I can at least find three reasons as why he should accept:
1) There have been several people who have been complaining about Oda focusing unnecessary on all these characters so much. Now if they become subordinate crews, all of that makes sense. They weren't meant to just spice up this arc, but will have another role to play in the long term.
2) What point was there for Oda to spend half a chapter explaining how they divided themselves into groups for Luffy then to decline the offer?
3) It's something a real pirate should do. In the end the entire Strawhat crew consists of not even 10 members. No matter how powerful they become, quantitatively seen they're really not worth much. Luffy needs some more backbone if they ever end up in large-scale situation.
At best Luffy might decline at the beginning because he hasn't a clue what they're talking about, but when Robin explains the benefits, he will make a 180° turn.