Game has local or online co-op for up to FOUR players. And it is an excellent co-op dungeon crawler. I am currently playing through with my kids, and we are having a ball. Be warned there is not loot and no levelling up. Each class has about 20 items for each of its 3 slots that are either sold at shops or found hidden in levels. A character's power is determined by what he has equipped, nothing more.
Not sure if there are still active players online, as it is sometime since I looked for any. The problem with playing online however is getting the difficulty right, and a team of like-equipped players. The host picks the level (from a choice of about 20), and the difficulty (from a choice of 5). The difference between Easy and Nightmare is enormous and it's pure luck who you get matched with, so it rarely deals you a hand that you can play. When I began playing I was often matched with people equipped with teh best 3 items, playing on Nightmare. Needless to say, I was simply baggage.
However, if you have multiple controllers and people to play with sitting beside you on the sofa, this is a great little game.