Board Thread:Dark Souls II/@comment-5264386-20131018105419/@comment-5264386-20140203073947

[Minor] Co-op Mechanics Update (3rd February, 2014)

 * Voice chat with other players is facilitated through wearing a particular ring. However, it would appear that voice chat only works with players that you've summoned, not with invaders (probably somewhat thankfully).
 * A ring can be obtained within the game which can be inscribed with the images of 10 different deities. Wearing this ring when inscribed allows the player to summon players who are wearing a ring inscribed with the same deity more easily, essentially facilitating easier co-op with friends.

Source
Dark Souls II: My First Hour - OroboroTheNinja