Now notice what happened to the barrel when Luffy proclaims his dream
First Oda makes sure Caribou is listening, then pushes the barrel near Luffy and makes Luffy proclaim his dream to the Crew. Waribou is definitely joining.
Foreshadowing all along
Forgot to Add that Caribou already asked to be Luffy's underling and Luffy accepted
Also Caribou is recommended to Luffy by none other than the God Raizo, a blessing which no other potential Nakama has.
Other reasons, Caribou is joining
1. Already intended to join SH crew, it is not his fault, they turn out to be fake SH
2. Lost a loved one, Gaburu's Grandma
3. Has X written on his shirt indicating he is the tenth.
4. Unique fighting style with Swamp logia, Scythe and Gattling Gun.
5. Is Actually funny enough to make readers laugh unlike Larrot or Lamato.
6. Prays to God to forgive the sin of attacking him and then crushes the Marine in his intro.
7. Forgiven by Luffy
8. Badass epithet Wet Haired Caribou
9. The crew needs a Crafty and Cunning person who can act as a Storage Room.
10. Also, Luffy calls Caribou by his actual name, not nicknames
Also Caribou has been keeping tabs on Luffy since Pre TS
As for Caribou's profession on the ship
Purser/Quartermaster