I am having trouble turning up anything besides the writings of Julius Caesar, Diodorus Siculus, and Tacitus on the subject. From what it sounds like, they were a lot like rabbis, regulating matters of British law which intersected quite a bit with religion, but like rabbis, they don't seem to have taken part in sacrifices - that was for an entirely different group, though druids would have been present during sacrifices.
So, does anyone else have any good sources?