User blog comment:Ricizubi/Do you believe that the 10th memeber is Shirahoshi? or Jinbei?/@comment-3075024-20110509201921/@comment-24.62.161.225-20120702031610

Jinbai has a dream... His dream is both Fisher Tigers dream and the queens dream. In order to fullfill that dream he must traverse the human world. In honor of Fisher Tiger and his Queen he must join with humans to create a new age were fishman work alongside humans.

Luffy already stated earlier that he wants 10 members for his crew. Whether he ment 10 members and one captain or 9 member and one captain is debatable. But it is clear that he will have at least one more crew member. Furthermore, if you look at Shank's crew there are 10 members. If you look at Black Beards crew there are also 10 members. Most likely the Straw Hats will end with 10 Members as Luffy wanted.

Spoilers.......

After the Battle in Gyoncorde Plaza, Jinbe offers to donate his blood to Luffy, saying that the law of Fishman Island which forbid fishmen to donate blood to human doesn't affect him because he is a pirate. A conscious Luffy then asks Jinbe to join his crew. While he turnd down the offer, he said that once he complete his task, he would meet Luffy and then Luffy can ask him again.

He declined due to his affiliation with Big Mom, but he also made a promise that once his unfinished buisness was done, he will return to the straw hat crew and join them. All the Straw Hats showed deep respect for Jinbe and wish him to join.

onepiece.wikia.com/wiki/Jinbe