The most prominent example is the Doobie Brothers: floundering 'til Michael McDonald joined and took them in a new direction.
pre-Michael: http://www.youtube.com/watch?v=kNTk9DKgnbk << not saying they had no meaning or sales, but they were mining an increasingly thin vein. Not just my opinion--their wiki makes this point too.
with McD: http://www.youtube.com/watch?v=t45QGRSXS1I
Assuredly there are other groups--care to contribute?