D'apres le forums officiel le multi est différent de Demon Souls c'est basé sur un système de p2p qui crée des lobby/servers et ce n'est plus un serveur dedié comme avant, du coup c'est totalement aléatoire et ca deviens impossible ou un gros coup de chance pour invoqué un pote.
"After successfully summoning, the client will return to their world if the area boss is defeated, the client is killed or the host is killed.
Also, please note that there are several different servers/lobbies that are being used for Dark Souls, so you may not be able to summon a specific friend into your game even if you adhere to the steps above. We can only recommend that you follow the steps above for the best chance at being able to join a specific friends game."
http://preparetodie.com/forums/viewtopic.php?f=18&t=2801&sid=e04ba5f4a8a422818ef93cd39e9caab1&start=35