Job Points, Levels, Experience Question

Want to make sure I grasp this concept while I'm still in the early phases.

Right now my main party consists of - Squire (Ramza), Archer, Black mage, Two knights.

Later, I would like Ramza to be a ninja or samurai.

How would I level his Thief? Do I have to have him AS a thief?

What if I have squire, but I have his abilities set as thief abilities (with JP Boost)?

Would he gain JP and job experience towards thief?

I guess the main question is, if I have a character with a certain job, how would I gain job experience in other jobs without having he himself that job.