I would say that Saumurai is joining the crew out of anyone considering he's following the exact pattern that everyone else followed (Robin and Brooke not included).
Upon first meeting him (or his legs rather), Luffy asked him to join the crew in which he said no to. He's tagging alone with the crew right now. He's probably going to be the one to fight Vergo I feel (how sad!!) and at the end his child is most likely not going to be in the New World and he'll have to tag along with the Strawhats.
Most follow this exact pattern as I said:
Sanji couldn't leave the Baratie due to giving his life to Zeff, so he said no, battled Gin / Pearl, and then (eventually) joined the crew.
Zoro couldn't accept becoming a pirate, Luffy kept prodding and asking, Zoro does battle with axe guy (I think??), and then joins.
Chopper said no, did battle with combo team of Wapol, and we got the greatest lines in One Piece "SHUT UP LET'S GO!"
Nami doesn't follow the pattern to a T, but she doesn't really accept the invitation to join the crew, she's just tagging along, she eventually decides to accept invitation to the crew.
Zoro and Luffy both assume Usopp joined the crew already after fighting cat man, but Luffy tells him to come along anyway, he doesn't want to leave his "crew members" but decides to go anyway.
Franky can't leave Water Seven, he helps the strawhats rescue Robin and fights what's-his-name, Iceberg + his friends tell him to go.
Brooke and Robin are exceptions, both asked to join the crew themselves. So cute :3
ANYWAY I like Samurai I want him on the crew stfu with your three swordsmen are too much, the crew needs a caretaker / father figure.