This covenant is actually required for two of the miracles you need to obtain. First off, you need to have at least 25 faith OR you can lower the requirement by 1 for each boss you help someone successfully kill. Once you're eligible, interact with the statue at the Sunlight Altar bonfire for the achievement.
To rank up in this covenant and to get the two miracles, you need Sunlight Medals. You can get Sunlight Medals by helping people to successfully kill bosses (WHILE YOU'RE STILL IN THE COVENANT), 3x out of the chest in the room by the second bonfire in Anor Londo (guarded by spear wielding Silver Knight), or by farming the sunlight maggots at the end of Lost Izalith. Offer ten and you'll get the Great Lightning Spear miracle. You can also offer Gwyn's soul for the Sunlight spear miracle.