No worlds left to conquer

[Rostum]

I quit when I reached level 34 and got to where the Mandragoras are and no one would let me join their party anymore.

That's because level 34 was too high for that area. You should have gone over to Altepa, which is a nice little journey from Bastok. Or seeked for a party from Jeuno, since the surrounding areas are good for that level too.