They were a vocal group, not a band. Sessions players backed them up in the recording studio and played behind them live, while they sang.
Their music was part of the soundtrack of my early teen years, starting with the one that made them a big name, 'Up, Up and Away'. There was 'Aquarius/Let the Sun Shine In', 'Sweet Blindness', 'Stoned Soul Picnic', and the Marilyn McCoo-dominated string of hits, 'Wedding Bell Blues', 'One Less Bell to Answer' and 'Last Night I Didn't Get to Sleep at All'. One of my favorites is the lesser-known 'Light Sings'. Great harmonies.