Unlike the Celts, we have a good idea of what Greek and Roman religious rites were, as the Mediterreanean cultures actually recorded their affairs in writing.
However ... things are not quite as they appear when it comes to those groups in Greece. They seem to take a rather Orphic view of things (The Orphics are a historically attested cult, but they were a mystical fringe cult at best). Furthermore, a lot of these Greek pagans seem to have a very Nationalist view of things, which means their religion is tinged with a good bit of ethnic politics.
A lot of us Greco-Roman pagans over here in the States have concluded those groups aren't really reviving the religion of ancient, mainstream Hellas so much as they are inventing a religion suitable for post-Cold War ethnic identity politics.
Nonetheless I feel they should have the freedom to do as they please. The Greek Orthodox Church has a frightening amount of restrictive influence over there.
PS - since this is about a modern group's slant on Ancient religion, rather than an actual ancient religion per se, I think it best to move this thread to the Afterhours forum.