I'm just going from what lil coop I experienced off of Demon's Souls. For coop - you drop a blue stone I think it was, anybody near that blue stone in another game can join your game to help you. so if you want to play with friends make sure you drop it
in a odd spot, call them up on phone since voice chat is not available and let them know where. if you defeat a boss that boss will remain defeated for you but not for the blue phantoms that helped you - joined your world. so basically host only gets
the progress but you share the souls, loot, experience.
You also can invade other players worlds as a black phantom. when you do this all enemies in the level will not attack you. if you kill a creature the souls automatically go to the person's who's world you invaded. so if you want to be a friendly black
phantom go pick off something difficult
Mostly though black phantoms invade for one purpose and one purpose only. to kill you. and if I read somewhere right, possibly steal an item from you.
there will be spells or items this time around that might let you appear to be some random object in the level, making it easy to surprise your victim. there will also be spells like the pyromancer has, I think its called gravelord or graveborn. either
way the spells only purpose is, it puts a monster in a random persons world and then continues to spawn enemies. so it's only meant to grief other players.
You can "curse" someone's game (it's random, not someone you target) by making enemies harder, and inserting Black Phantom NPC's into the game. In order to remove the curse, you need to find the spot where the other person cursed you, and invade him (you
can bring coop help with you), and kill him. Aside from that, it's mostly what Husker described. Dropping a stone of some kind, which others can pick up to summon you.
In Demon's Souls, the upside of being a coop helper is you don't lose your souls when you die (unless a BP kills you). If you're new to the game, I strongly advise you to do lots of coop work when you start. You'll learn about enemy tendencies, the layout
of the world and the traps, and get to see how other people deal with enemies. All that time, you'll be levelling up or earning souls to upgrade weapons. It will ease the transition for you in your own game.