Luffy hasn't really bonded with him, their only encounter was when Bartolomeo built those barrier stairs for Luffy and when Zoro sent Bartolomeo to get Luffy for him. Luffy doesn't even know his name.
Luffy hasn't connected with either one of them enough to invite them.
Luffy never really bonded with brook and he invited him. All luffy needs for a invititation is someone to interest him enough to invite them. There was even a point were luffy invited a bunch or people(or things) and his crew got upset at him(it was part of a gag though)
So just because he hasnt bonded YET, does not mean that he wont. All it takes is maybe just a scene, or just luffy finding out what they did for him to invite them. Luffy pretty much is cool with anyone who is not evil joining his crew.
The biggest determining factor about a crew member joining is not being invited. It is them accepting the invitation, or wanting to be a part of the crew, and them having some sort of dream.
Heck robin never recieved a invitation PERIOD and she joined.
No one reasons:
Bellamy: Fodder
Rebecca: She just got her father and her family back why would she leave them also kyros said he won't let her fight...
Viola: like Rebecca she just got her family back
Bartolomeo: HE HAS HIS OWN CREW.....what do people not understand about that???
Nami is fodder, ussopp is mostly fodder. Yet they both were invited. A members strength has little to do with them being a member. It is more of the role they serve in the crew. And that they have a dream.
For starters bellamy while not especially strong, is not fodder. Secondly even if he were, that would not stop him from joining.
Rebbecca, in my opinion, has zero reason or purpose for joining. I see kyros as the new king, and rebbecca as the princess.
Finnally the reason i replied to your post.
Let me make a statement.
Ace was never part of whitebeards crew. Because he couldnt join, because he already had a crew. Its impossible for a captain to join under a stronger captain.
That sounds stupid and is obviously false. Ace joined whitebeards crew. He left his crew behind and moved on.
There is no reason barto cant just join luffies crew. His "crew" is very vague and undefined like aces crew was. And really his crew is just a luffy fan club.
Sure having a crew makes things less LIKELY. But it is far from the deciding factor.
I mean technically ussopp had a crew. Franky definately had a crew.
Anyways the moral of my replies is that it is STUPID, to just assume a character cannot join because they are "fodder", or because they have a crew. Both things can support a arguement and perspective as to why joining is unlikely. But it doesnt just make it impossible. Especially as we are debating actual likely canidates(as opposed to pretty much impossible canidates like trebol joining straw hats)
Anyways now time for my thoughts on the canidates.
Rebbecca. At this point she seems to have less combat potential than nami. She has not done anything in combat, other than dodge. Even pre SH ussopp and nami had more fighting feats(ussopp with his slingshot, and nami with her staff) She has no real traits that make her stand out as a canidate at all currently. Sure something could change, but I just dont see it.
She is a 2 chance of joining. 0 being no chance, 1 being very unlikely, 2 being unlikely, 3 plausible, 4 being likely, and 5 being very likely. 6 would be this is a new future sh member.
Viola is a 3, . She is a plausible canidate to join. Her power is useful, and unique, and she would bring a interesting dynamic to the crew, especially with her interactions with sanji. She has some fighting abilities as well. She could fill the role as a spy/intel gatherer in the crew.
The main thing barring her from joining, would be she does not have a dream we know of, and so far there is not a spectacular motive for her to join. Also ussopps unlocking of long range sight(his haki) further decreases the chances of her joining.
Rebbecca is between 1 and 2. Right now the only thing going for her is her interactions with luffy. If this arc is really similar to the alabasta arc, then she will be like vivi. The failed recruit. She does not have the offensive capabilities that vivi or any other sh/sh canidate has shown. She is purely defensive.
Meanwhile you have kyros not wanting her to ever fight again, which further decreases her chances. So unless she has a change from now to end of arc, I just dont see her joining.
Barto. Barto is a 4. He has strong reasons for joining the crew, a useful power, and he has good interactions with the crew.
However he does have a crew/group, and he has the fact that he is somewhat copying luffies moves(see bari bari pistol). Also his dream is not entirely clear.
I do see however that he could easily join.
Bellamy. A 4. There have been numerous comparisons of dressrosa to alabasta. This would be one of them. Bellamy essentially would be a robin. A enemy that became a friend. A enemy that would have their everything destroyed by their leader falling. That would not have anymore reason to live.
Luffy clearly has shown he cares about bellamy.
However it is not clear whether bellamy has a dream, and how his interaction and role in the crew would be.
Kinemon I think is a 4. He already fits in well with the crew, and his swordsman ship is unique enough, also he has a useful df for the crew. Disguises would be especially useful now that luffy and crew are well recognized. He would also fill in the theme of swordsmen. Brook as 1 sword ice style, kinemon as fire 2 sword style, and zoro as mid range/close range 3 sword style.
However he would not join till momo is safely home. He would be a companion like vivi until he becomes a official part of the crew(if he were to join). The main thing against him is the lack of a dream so far.
Caesar 1. There is a chance of him joining, and he would definately fit the role of scientist. However there is just no special compelling reason for him to join, and he has done some quite horrible things. He just doesnt fit in the crew of "nice" pirates. However there is always the possibility of change of heart.
Jimbe 5. He really should be a 6, as he has already accepted the invitation to join.
I am convinced that oda is just delaying his joining till after having m3 grow to be clearly stronger than him. Unless jimbe is killed, I think he will join. Also he has a dream of seeing the fishmen be regarded as equals. The strawhats have lots of variety, and a fishman would fit right in. Also he could fill the role of tactician.
Smoker/tashigi 1. I have read some interesting theories about how one of them could join up. They would fit the role of ex marine of the crew. However smoker has not shown a dream, and tashigi while having shown a dream, does not bring anything special to the crew.
Bonney. 1. She would fit in as a tomboyish female compared to robin and namis feminine attitudes. She could be a rival for luffy in eating. However there is no dream shown, and so far no special motive for her to join.
These are my opinions simply based on what we have seen so far. When things change in the story they change the opinions on what I think will happen.