Luke Smith from Bungie has revealed in a fresh interview that the six man raids in Destiny are purely friends only, with no added matchmaking.
Bungie has not said much about these raids before, and they should not be confused with Strike missions. According to Smith, these raids can be expected to take a couple of hours, and they certainly require teamwork and communication between the players. It is due to this that Bungie does not want to involve random groups in them.
Smith promises that these raids "demand of you things you've never even really been asked to do in a shooter before." Sounds spooky.
Smith also noted in this same interview, that the endgame in Destiny already begins in part at level 20, when the player sees how the first part of the adventure ends. After that they will be coaxed back with activities such as these raids, as well as daily and weekly events.