The Police:Sting
Andy Summers
Stewart Copeland
Klark Kent
Well now we’re reeeeally stratching the definition of “successful.” Managing to produce a solo album that sells 10 copies doesn’t cut it, IMO.
Joe Walsh had a career before he joined the Eagles. Hard to say if Timothy B. Schmidt or Bernie Leadon had a career after the Eagles. 
Satch had a career long before Purple. Coverdale is a good mention. Steve Morse went into Purple with a string of acclaimed albums both solo and as the leader of the (Dixie) Dregs.
Rockpile were a strange case. Because Lowe and Edmunds were signed to different labels, they couldn’t record under the name Rockpile. The band appeared on albums by both Lowe and Edmunds until the conflicting contracts expired. They made one perfect record and then split up.
Well, with apologies to Astorian, Biffy, et al, I’m as big a Crimhead as anyone on this board. But even I wouldn’t argue that someone who moved from Yes in their heyday to King Crimson could be considered to have become popular because of King Crimson. And Adrian was with Frank Zappa, then Talking Heads and Bowie before being a Crim, and Tony was a much-sought-after session musician before Crimson as well…
Robert Fripp and Mel Collins, maybe?
What about successive lead singers for the same band who then left for solo careers? I ask because of Black Sabbath: Ozzy, Ronny James Dio … perhaps one or two others?
How about Santana? Besides the eponymous Carlos, we have:
Greg Rolie & Neal Schon - Journey
Buddy Miles - although he was established before Santana.
Coke Escovedo had several albums after he left the group.
Curtis Salgado - Robert Cray Band, Roomful of Blues, and a pretty successful solo act.
I’d say Santana fits the bill!
You know, I absolutely adore Tommy Bolin and I’m trying to figure if him being with the James Gang is simply something I forgot, or if I just followed him from a later point and never checked his history. Regardless, RC, thanks for that. I’ll be trackin’ those two JG albums down. Oh, and DP’s Come Taste The Band was sweet but Teaser (minorly) and Private Eyes’ Post Toastie (major major majorly) just slay me.
That’s fine, but you missed the second part of my question: what has Brett Lunsford done? Are you talking about his work with D+?
Going the soul music route:
The Impressions–Jerry Butler and Curtis Mayfield
The Dominoes–Clyde McPhatter and Jackie Wilson
The Drifters–Clyde McPhatter and Ben E. King
The Soul Stirrers–Sam Cooke and Johnnie Taylor
(The Rhythmatist is available starting at $43? Holy crap - damn glad I bought it back then…)
Jeff Beck is one of the most respected guitarists of all time. He’s won 4 grammys and has a host of gold records as a solo artist.
He’s kinda rsponsible for launching Rod Stewart and Ron Wood (both were in the Jeff Beck Group) but I don’t think we should hold that against him.
When i think of Buckingham’s solo career, the first song that comes to mind is “Trouble.” I think it went top 10 in the US.
I never knew he sang the “Holiday Road” song from the movie.
Doesn’t really count; Buckingham was hired (along with Nicks) because he (they) had already had some success as “Buckingham Nicks.” Mick Fleetwood hired them to spice up Fleetwood Mac. So they’re kind of the opposite of the OP.
If that’s the definition, then ABBA counts, with both Friday & Agnetha releasing pre- and post-ABBA albums that sold a bit.
[TotalFloydNerdNitpick]Unless you count British pop charts, with which the Floyd had a #1 hit with See Emily Play[/nitpick]
Hardly the opposite.
The OP wanted “well-known groups that spawned more than one popular solo artist.” The opposite of that would be well-known solo artists that spawned a popular band.
Neither Nicks nor Buckingham was a real solo artist (well-known or otherwise) before joining Fleetwood Mac, and they certainly didn’t spawn Fleetwood Mac, which had been a popular band, especially in the UK and Europe, since the late '60s.
Also, the Buckingham Nicks album was hardly a raging success. Both artists only gained real exposure and fame with their invitation into Fleetwood Mac. Then, after Fleetwood Mac, each had successful solo careers.
I don’t think it’s a stretch at all to fit them into the OP’s formulation.