The angels were stationed on Earth thousands of years ago to watch humanity. They took part in the massacre in Ancient Egypt under Naomi's monitoring. They laid siege to Hell to help rescue Dean Winchester. Uriel was promoted to leader of the garrison and killed seven members. The Prophet protectors were killed by Leviathans.
In the box set art for Season 4 DVD set and the cover for Season Four The Official Companion Guide, behind Sam and Dean, are Castiel and shadows, the shadows likely representing his angelic garrison, who assisted in rescuing Dean from Hell.
