“Does it count when they were the Stone Poneys, backing up Linda Ronstadt?”
No, because none of the Eagles were in the Stone Poneys. That group was an earlier backup band in Ronstadt’s career, in the 1960s.
The Eagles, or the ‘proto-Eagles’, were recruited by Ronstadt as backup for a Disneyland gig in 1971.
Frey played with Bob Seger back in Michigan in the late 60s.