Maurice White (one of the singers in Earth Wind and Fire) started his career as a drummer for Chuck Berry and Booker T and the MG's. Ronnie Laws played sax for EWF before starting his own solo career.

Bobby Womak played guitar for Sam Cooke in the early 60's till Cooke died.

Al Green had his own band, The Creations, who would later become Al Greene and the Soul Mates (he dropped the 'e' at the end of his name after the group broke up) which had his first hit, Back Up Train

Boz Scaggs played in Steve Miller Band.

I'll try to remember more later.