Complex sentence: Each had left a band to join forces and, after going through several drummers, they settled on Johnson, as well as replacing lead singer with Gregory Walker, who left the band just prior to the successful release of "Boogie Oogie Oogie".
Split into simpler sentences: Each had left a band to join forces, and after employing several drummers, they settled on Johnson. They also had replaced the lead singer (unnamed) with Gregory Walker, who left the band just prior to the successful release of "Boogie Oogie Oogie".