I don't know how much has changed but on PS3

Blue Eye Stone messages which allow you to summon players in phantom form will appear and 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.)

^This is a lot different than co op in Dark/Bloodborne