Complex sentence: Stein and Bogert played in a local band called Rick Martin & The Showmen and were so impressed by the sound of The Rascals (swinging and floods of organ) that they decided to form their own band with Martell and Rick Martin's drummer, Joey Brennan.
Split into simpler sentences: Stein and Bogert played in a local band called Rick Martin & The Showmen. The pair were so impressed by the swinging sound and floods of organ of The Rascals they decided to form their own band with Martell and Rick Martin's drummer, Joey Brennan.