There as been a great deal of confusion regarding this last faction of the Horde Expedition. Specifically who they are and how they came to be there. The Sunreavers are a group of Blood Elves who petitioned the Kirin Tor for the acceptance of the New Horde. This is HUGE as the Old Horde was responsible for almost completely destroying the city of Dalaran during the Second War. While the Kirin Tor and Dalaran specifically have attempted to remain neutral during the various conflicts that have plagued Azeroth that does not mean that they have remained uninvolved observers. This dual citizenship conflict is most recently played out in the form of Archmage Aethas Sunreaver.
Archmage Sunreaver is a member of the Council of Six, the ruling body of the Kirin Tor. He took the position Prince Kael'thas abandoned following the destruction of Silvermoon City. It has not been an easy transition and his presence has not been welcomed by everyone, most notably his cousin Vereesa Windrunner, who happens to be the spouse of the Kirin Tor's present leader Rhonin. However he has persevered and though his efforts managed to establish a sanctuary for the Horde within Dalarn itself. His supporters have taken the name "Sunreavers" to honor him and his tireless efforts. Another thing of note, at the beginning of the expansion Aethas was not wearing a tabard but that was later changed, while he still is affiliated with the Kirin Tor he wears the Sunreaver tabard.
Like their Silverwing counterparts players were teased at the beginning of the Wrath expansion with a faction bar but no way to gain reputation. This was rectified with patch 3.1 and the introduction of the Argent Tournament, here the Sunreavers play host to the Horde forces. Originally the plan was for the Tournament to take place in Crystalsong Forest just under Dalaran, as a result small bases were constructed in the zone complete with flight paths. However Blizzard soon realized that with all the activity taking place overhead it created to much lag so they relocated it to Icecrown.
Horde players can now gain reputation with the Sunreavers in two ways. By doing the daily quests located at the Argent Tournament or by running level 80 dungeons while not wearing a reputation granting tabard. Once you have reached Champion with at least one of the home factions as well as exalted with the Sunreavers additional daily quests will become available to you. More importantly players will be able to purchase the flying mount, land mount, pet and tabard for their various collections.