C'mon people. Jinbe is a man of his word. If he said he's gonna join, he will, no matter what. It's not so hard imagine he's gonna join at some critical fighting point in the future. So we can consider him a member.
As much as we can consider Vivi :ninja:
–- Update From New Post Merge ---
Btw, about the whole Luffy will protect Shirahoshi thing. If the World Government, meaning the Gorousei, find out that Shirahoshi is Poseidon, do you think Jinbe and/or the SH can really protect her? We have seen how afraid the Gorosei are of everything concerning the Ancient Kingdom and the Ancient Weapons. They would immediately send out their strongest fighters, be it the marines with VA and Admirals or be it Cipherpol Agents. Luffy and his crew wouldn´t stand a chance against this power.
First of all, I don't see how anyone will find out she's the Poseidon for now if only Robin and Neptune know that.
Second - he did protect Robin, didn't he?
Third - There's always Sea Kings…