It isn't coop at all, AFAIK there is no way to specifically play with friends (I know this was true in Demon's, I never looked into it in Dark). And in order to play with other players at all you are opening your game up to invasion by other players (forced PVP is not my cup of tea).
I loved the core of the game but some of the design decisions and rough edges really, really, REALLY rubbed me wrong. While it was fine overlooking them for the duration of Demon's they kept me from playing Dark for too long. Anyways Dragon's Dogma eventually came out and offered the same style in a less punishing package with more fleshed out combat/classes and best of all NO FUCKING PVP.