Celestial Overseers are part of Universe Aids group.  They appear named in Uversa personality register. 
Before leaving the universe of Nebadon, most Urantia mortals will be afforded opportunity to serve for a longer or shorter time as members of the Nebadon corps of Celestial Overseers. 
There are upward of three million of them in Nebadon, and they are all volunteers who have qualified by experience to serve as educational advisers to the entire realm. 
See also: UB 37:6.