http://www.squarebooks.com/faulkner/chron.php
This is a chronology of Faulkner's life from one of the bookstores in Oxford. They have him joining SAE in 1919, which would have been when he was at Ole Miss.
It seems like they'd be up on their Oxford history.
So maybe he was a regular member.
ETA: why do I only see my errors when I see them quoted? Why can't I proofread my own comments the first time? Argh.