Slender Source mod will allow you to play with friends 

For those who are too afraid to go it alone in Slender Man, a work-in-progress version using the Source Engine is in the works which will allow you to play it with equally masochistic friends.Here's the blurb: “Players must work together to collect a certain amount of creepy dolls all while trying to avoid the abomination that is the Slenderman. The kick? No weapons.” You can follow the progress of Slender: Source over on ModDB. Thanks, PC Gamer.