1) For the main character you gain new persona by either earning them after battles (via shuffle time;a bonus game that occurs after battle) or by fusing other persona cards together to make new ones. (Make sure you register your Persona in your compendium in the Velvet room, that way you can always summon a persona that you've owned before; it just costs money).
2) They level up like the main character does... whatever Persona you have equipped when you gain exp, that persona will also gain exp... There is a notification on your persona status screen that shows when the next spell will be learned(ie; next lvl 15, etc)... Each persona can only have 8 abilities at a time, when another one comes along you will be asked to discard one.