Michael McDonald was really great in that role with that interesting "horn" kind of vocal tone when he did backups. Like having a backing vocal and horns.
The Bee Gees on Heatbreaker Chain reaction Woman in love Immortality amongst others (they wrote them as well)
I saw Sting guest on Money For Nothing when Dire Straits played Wembley Arena the same day that Live Aid was on at Wembley Stadium. Their rhythm guitarist was quite good too... I'm sure Sting's contribution was a major factor in making MFN such a big hit. It's a bit beyond 'backing'.
Timothy Schmit on a lot of tunes including this one. All of those "Oh I Love You" responses and those in the outro are Timmy.
How sad was it back when Sheryl Crow was the backup singer for Pedo Jackson? Pedo Jackson - I Just Cant Stop Loving You