GreywolfJulius Caesar is one of only two ancient writers who actually knew some Druids. They told him that Druidry originated in Britain and that Druids from the continent still came to Britain to study it in its purest form. Since Celtic culture developed in Central Europe, this means that Druidry was not Celtic but developed in Britain before Celtic culture reached here in around 800 BCE. This makes Druidry at least Bronze Age, possibly Neolithic and definitely British. So it is possible that Druids were around during the building of Stonehenge.
As for what ceremonies they may have conducted there, we can speculate based on the archaeology of the place, the various alignments built into it, what we know of the religious practices of the time from elsewhere in Britain, how other cultures use stone circles and our own visionary experiences.
I think some of what we do there now would be broadly familiar to those who originally used the site. Same goes for Woodhenge, built at the same time and by the same people.