The Jaynetts where really more of a studio concoction than a real group. In fact, the three women usually in the Jaynetts publicity photo, may not even be the girls that recorded the song.
There where at least five female vocalists in the studio of the "Sally" recording session.
The distinct sound of the hit single was created by Artie Butler, the arranger, whom played the base rhythm track on piano. While mono-to-mono dubbing, he added another piano, an organ and a bass guitar, all played by himself. Drums were finally overdubbed over that again (by some session dude), the resulting sound had enormous ambiance.