The Host Player and a Blue Phantom. I think they're trying to surrender.
Passive Co-Op
If you have been playing online you may have noticed a few peculiar things as you move through the world. You'll see the ghostly images of other players passing through your game. The effect can be unsettling at times. You will also see bloodstains. Lots and lots of bloodstains. These are where other real life players have perished. When you touch their bloodstain a translucent red ghost will re-enact their final moments in real time in your own game. These displays can signal a precipitous fall ahead, a devious trap, a hidden foe, or other dangers.
Players can also leave messages for one another from a list of pre-set phrases. You may see a message that says "Use fire on the next enemy" or "Beware of false messages." Yes, people will mess with you, even with the simple message system. Players can recommend any message they come across. Each time they do, the player who wrote it will recover health.
Active Co-Op
Once you've defeated the first boss you will get your body back. Your life bar will be fully replenished, and you'll be free to adventure in the horrible, demon infested land. The pathways of the hub world will open, and you will be free to explore the rest of Boletaria. There are five specific areas, each with it's own enemies and dangers. You'll also receive the Blue Eye Stone, which will let you play the game cooperatively.
Not so fast! There's a catch. Actually, there are lots of catches. Demon's Souls has a very unique multiplayer experience. People either love it or hate it. I've found very little middle ground. How you go about playing co-op depends on what "form" your character currently has. You can play the game in either Body Form or Soul Form. You may only co-op with players who are close to your level range. So you can't play with a level 500 player when you yourself are only at level 23.
A player in Soul Form in the Nexus hub.
When you are in Body Form you will have a full life bar. You'll do a little less damage than you would in Soul Form. Blue Eye Stone messages will appear throughout the world. They'll most likely be located at the beginning of an area or right before a boss fight. These are markers that other players have left, offering their services. When you examine a Blue Eye Stone you will see a player's name, how many co-op experiences they've had, and their co-op grades. (At the end of each co-op session you will grade each other's performance. Souls earned from the final boss battle will be divided amongst players according to the grades they are given.)
While in Body Form you can summon up to two other players to aid you. You will act as the Host Player. All game progress will occur only in your world. Summoned players will enter your game as friendly Blue Phantoms. These Blue Phantoms can kill enemies and interact with the environment (throw switches, break objects). They cannot pick up any items in your world unless you drop them from your own inventory. They cannot interact with the few NPC's you'll come across. You will both earn souls for defeating enemies. There is no form of in-game chat. You can use the tedious PS3 messaging system, but without the ability to pause the game, that is ill advised. The three of you may communicate through a few gestures from a menu you can pull up by holding the 'X' button. The gestures are essentially useless, and totally unwieldy in battle. It's pretty much understood that you'll be attempting to fight the boss together, and the Blue Phantoms will generally take your lead.
This unspoken communication gives the co-op an interesting feel. When played solo, Demon's Souls has an oppressive feeling of loneliness. Once your silent partners arrive, there is little you can do to express your will. You know that you each share a common goal, and the cooperative gameplay is fueled by the need to stay close to each other and work in tandem to defeat enemies. As the host you can dismiss uncooperative Blue Phantom players if you wish, so they usually stay close and help out. Everyone wants a piece of the Boss Demon soul smorgasbord, and to do that they need to earn high grades from their peers. Blue Phantoms need to be the best co-op player they can be. And so does the Host, for that matter.
If you take on multiple enemies at once, you are going to die.