Client: Ghebi Damomohe (Neptune's Spire, Lower Jeuno) [I-7]
Talk to one of the NPCs in the only room you can enter and he gives you a (cryptic) hint.
Then talk to Ghebi and move the selection cursor down to the bottom until you have selected a blank option.
Ghebi will then give you a Tenshodo Mebmership Application form.
Go to the Warehouses in Port Bastok and speak to the NPCs there to finish the application form.
Return to Jeuno and speak to Ghebi.
You will get a membership card, and a Tenshodo Invite (to give away or sell).
Or you can buy it from AH or get ask someone that has one.
Tenshodo Member's Card