I see and hear that in this clip Doug Hastings from the band Daily Flash replaced Neil Young at Monterey Festival . If Monterey had been two months earlier, it may have been different. Doug is playing the harmonics and the lead guitar on this tune, the Daily Flash who were popular and well known locally (west coast; San Francisco up thru Vancouver BC) and often billed as an opening or middle act in the hey day of the San Francisco concert / ball room years.
https://www.youtube.com/watch?v=cNdljO4cTa4
Stills and Crosby were already friends from back to the Whiskey gigs, they had really been close during those past weeks or so just before Monterey. In fact I think the Rock and Roll Woman sessions were going on just before and after the Monterey weekend and Crosby was in there for those too since he came up with the riff idea. With Neil Young gone and Doug a not-so-sure-bet as a replacement, and everything pretty ragged with the band anyway, hey, why not have Crosby jam in. He was up in Monterey for the weekend too so why not.The other guys are playing rhythm and singing.
LIne-up: David Crosby guest who sat in the whole set, Richie Furay,Stephen Stills,Bruce Palmer,Dewey Martin,Doug Hastings on lead.
The Fat Angel Sings 