The Sunreavers is the main horde faction for the Argent Tournament. Doing almost any quests at the Argent Tournament will raise your reputation with The Sunreavers as well as the other "Horde Expedition" factions.
When you reach max exalted with The Horde Expedition you will notice your Sunreavers reputation start to increase when running Northrend Heroics. The Sunreavers have a Quarter Master that is located at the Argent Tournament Grounds in Northern Icecrown.
Raising Your Reputation
The best and fastest way to raise your reputation with The Sunreavers is by completing the dailies at the Argent Tournament Grounds. The second best way to raise your Sunreaver reputation is by running heroics without a faction specific tabard on. However, usually players will do a mixture of the two.
Below is a list of both sets of dailies available to you via the Argent Tournament Grounds. In order for the Champion Seals dailies to become available to you, you will first need to become a Champion of a city. To do this you will need to collect 25 Valiant Seals and turn them into your reputation specific NPC.