Important additions

Most were thinking of compenents. Hah! I'm thinking about most important addition to a band. Just watched Pink Floyd in St. Tropez with David singing "Green is the Colour". Just wonderful. Anyways, he had me thinking about which add on artists to a band were most influential. I thought of 4. David Gilmore with Pink Floyd. Phil Collins with Genesis. Ringo Starr with The Beatles. And Brian Johnson with ACDC. However, who are the less popular artists that had a big impact when they came on board?
Without a doubt the most famous addition was when Blue Oyster Cult added more cowbell.
+1 MC

Young added to CSN. I know, Young wasn't exactly unknown, but best I could come up with. I can also add a worse, Bob Dylan with the Dead. Godawful.  
Rick Wakeman-Yes.
Ian Gillan-Deep purple.
Neil Part-Rush.
Tony Levin-Peter Gabriel.
Carlos Alomar-David Bowie.
Eric Clapton-The Yardbirds.
Jimmy Page-The Yardbirds.
Peter Baumann-Tangerine Dream.
Steve Hackett-Genesis.

Not a lot of Country Western lovers here. What about when Johnny Cash added June Carter Cash? And then the topper, when Dewey Cox added Darlene.
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Without Peart coming on board, Rush would never have been Rush. And without Rush, rock music wouldn’t have been the same.

Also, Buckingham and Nicks joining Fleetwood Mac
@wturkey - thanks!
Also, when Mozart joined Salieri, that was pretty important