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?