It struck me that I don't think we've ever had a topic about this - precisely because to my knowledge there were virtually no guest artists, apart from David Sanborn's sax on The Long Run. There was also a Celebrity Chorus on The Greeks Don't Want No Freaks.

But I've just done a quiz on Facebook & one of the questions asked 'who co-wrote & sang uncredited backing vocals on Heartache Tonight'? The answer is Bob Seger, of course. Has anyone ever heard that he sang backing vocals on the song?

For what it's worth here is EOC's list of additional musicians. It's notable that there really aren't any big name guest appearances.

https://eaglesonlinecentral.com/band/index.htm