Probably shouldn't count, because he didn't really leave his solo career--they were concurrent, but Todd Rundgren with Utopia. Especially the later 70s/80s version of Utopia Todd wanted to be a democratic band.
Paul McCartney with Wings, although it's arguable just how much of a band they were at times
Robert Palmer with Power Station
Wasn't Todd Rundgren successful with Nazz before starting his solo career?
Stevie Nicks and Lindsey Buckingham
I am trying to think of successful duos who joined a group