Glen Campbell was a member of the Wrecking Crew before becoming a major recording star.

In the early years of her musical career, Cass Elliot toiled in the folk scene with a couple of rather nondescript (at least at the time) folk groups, The Big 3 and The Mugwumps.  Then she got her invitation to join The New Journeyman, who changed their name to The Mamas and Papas.

Before they became successful singers on their own, Sonny and Cher worked for Phil Spector - Sonny as a promotion man and Cher as a backup singer.