You can pretty much play the entire game with friends if you want. I sold the game a while ago, though. It was good, it's just that it was too time consuming which is something I lack right now.
There's not really any traditional coop, unless you delve into the online (which is the meat of the game), which can't be played split screen. The only split screen coop is through the arena, which are single fights against boss monsters where you choose from 4 predetermined sets of weapons/armor/items. Also I hear the HUD gets kind of slaughtered by the split, which is pretty crucial to playing properly, but I can't say for sure.
Still, if you have friends you can play online with, it's a blast.