How can I join a friend for co-op?

To join a friend, one of you must be alive, and the other must be dead. Also (at least to my knowledge) neither player can have beaten the boss of that level. Meaning if you have killed the tower knight, and he has not, you cannot join him.

Aside from that, all you have to do, is have the player that is currently dead, use a blue stone. The will place a marker on the ground that the living player can see. He can then attempt to join your game, and the dead player will get a message on whether to approve or deny the entry of the person.

In response to MSDSS, I didn't get a message asking of approval, it just went straight to being summoned... which sadly it wasn't my friend who summoned me.

To join a friend they have to be within 10 levels of you, and one has to be alive while the other one is dead. If both of you have beaten the boss of that level you can not join each other, that is true, but if one of you hasn't beaten the boss you can join that player. (only if he's alive and your dead.)

To join the player, the dead person (Phantom) must have a "Blue Stone". The Blue Stone will allow him to place a marking on the ground that a living person can see. The living person then get's a choice to either summon them or not. (Up to 2 blue phantoms only).

If your trying to join a friend you want to have them(or you) place your Blue Stone somewhere where you think no one else would find, or you can wait for your friend to meet you in a certain spot. Message each other of where to meet and when to place the stone. Sadly, there is no voice chat.

Hope this helps. (You probably figured it out by now though)

Yes you can, but watch out for any Red Eyes out there, they can steal your level.