Co-op is pretty simple. You create a room that 2 others can join and they control the party members during the missions. All healing spells are dungeon wide and killing enemies with the soul burst increases everyone's MP bar too. You revive party members with potions or they can use a shared stock of Phoenix Downs to revive themselves. Everyone has their own loot, so you don't gotta worry about someone being a loot black hole either. Any party member not controlled by someone else just functions how it does normally in solo play.
Short game overall but I can see why some people just keep playing for the fun of it. And I can def see why casual/story mode exist lol.