How would we know?
And more importantly, who cares?
with great power comes great responsibility.
I am pretty convinced that there are loads of exclusive societies in Oxford, Assassins, Stoics, The Grid, Apollo, Piers Gav. To name but a few and membership is not massively secret I don't think.
Obviously Cambridge has mirrors of each of these. I am certain it does for at least one as I have had dinner there a couple of times for the sake of reciprocity.
Years ago, my brother and his friends heard about some secret society meeting in Oxford. I don't recollect much of it, except that they climbed on to a wall to peer into window (which means that it must have been some other college apart from Pmb), were spotted and then ran for their lives when chased. I recollect him saying that it made him quite uncomfortable & he really isn't a chicken (he's a 3rd Dan & the other two had stints in the army)