You need to register them in the compendium and you can summon them any time you like, for a fee. No need to say goodbye!
Gonna finish the castle then get stuck into some fusion tonight, haven't done any of that yet. Is it worth levelling the personas up a bit before fusion or just get on with it? I thought I'd wait for a while but then I had to throw one away when I collected another one so I guess is better get on and fuse. Be sad to see some of them go ;(
p.s - i just tried the final optional boss of the dungeon and he absolutely owned me. 1 aoe rampage and the entire team dies.
PSN : Gregolution
XBL : Gregolutionz