Dungeon queues haven't been put in so far, because it gives an incentive to socialize. ;)
A good way to queue for a dungeon is typing:
/join main
/main Hey I'd like to do the Hollow Tree, can anyone help?
;P
I don't think this is a good idea, because if ONLY damage or only heals queued, and 5 is the limit for a dungeon, the dungeon might not go well, so...
The trials do have a queue, and there it simply doesn't make a group with five damage dealers. If five damage dealers queued and no one else, it'd make no group. If five damage dealers, a healer and two tanks queued, it'd make one group with three damage dealers, one healer and one tank, and leave the rest queued.
Still, people don't usually use queues. Whenever people do runs, be it trials or dungeons, it's usually because they talked and were like "oh, let's do something together" - not because they spontaneously queued. ;P