Actually you dont even have to do that. Apparently an already made group can do a randomly assigned dungeon for the same rewards. That was posted by a blue today, sounds like the heroic Daily Dungeon quest, but maybe its a repeatable one?

edit: quote.

Quote Originally Posted by bluetwitterchatfromtoday
Q: May I create a group with 4 friends and then use the LFG interface to gain the random dungeon reward?
A: Yes. In fact, you can join it with a full group of 5 and still get the random dungeon reward. The reward is for doing a random dungeon, not necessarily for having random members.