Unlike the other factions, "The Dawn" is not so much an organization as an allegiance, to the Avatar and keeping peace in the world.
It started as a nickname for Aang, calling him "The Dawn" since he was the beginning of balance restored, the bringer of an era of (relative) peace, the reason for it all. Therefore, it automatically includes many of those we know from the old gAang. It also has become an umbrella term to include any and all who would answer the call to war by standing at the Avatar's side. Even those who have never met the Avatar or his friends personally might fall into this group, so long as they would oppose the other factions if a confrontation arose.
Given the notable figures in this alliance, The Dawn is a force to be reckoned with, home to many master benders and some of the most powerful nonbenders of the age. The Order of the White Lotus is the real organization behind this faction, with Aang as the "leader" of this group of people. That's not to say the OWL is not as secretive and exclusive as it ever was--it is. They simply form the backbone of this wide array of colorful characters, maintaining their own independent organization in the four corners of the world.
For all intents and purposes, this is the "good side" of the game.