Hint: If you want to play with a friend both should be in one of the 3 Cooperative Covenants (Princess's Guard, Way of White, Warrior of Sunlight). Then you can place your Summon Sign with your (White Sign Soapstone). If your friend doesn't see the sign, you just have to place it again. After a few times it will eventually show up in your friends world. It seems that the server is not important but the sign will only show up in a couple of worlds. So you have to place it some more times until it shows up in the designated world. To note is, that the sign will almost instantly appear in the other world you don't have to wait long until it becomes visible to other players. This was tested a couple of times and it never took longer than 5 minutes to find each other.
I've searched and found that several people are saying that this works pretty consistently. Also that it takes somewhere between 10 seconds and 10 minutes.
HKS, it's not really that hard. To SUMMON someone, you have to be human form, and the boss of the area needs to be alive. To BE SUMMONED you can put your sign down anywhere, under any conditions. You just need to make sure your level is about the same. You can co-op in the area after you defeat the boss, but only by being summoned by another player.
So I could, in theory, clear out the entire game at a low level (I've done a level 1 playthrough before), then just let someone summon me in the first zone, move on to the next one instantly, let them summon me again, and level up with them. Yes, it's absurdly complicated to play the entire game start to finish with one friend, but that's because they didn't want the game to be played that way. Arranging for co-op in one or two zones would actually be pretty easy.