Jimi Hendrix. Jimi was playing the amazing lead guitar on the track while Stephen was on the Organ. Kudos to Gerard who once again posted a comment with the correct answer!
As Gerard pointed out in his answer, Jimi was not the only famous guitarist that sat in on Stephen’s first album. Eric Clapton played a second lead guitar part on an album track called G Back Home. Let’s listen….(Eric starts to show up around 3:40 as best I can tell and then all bets are off!)
There was a pretty interesting group of backup singers on this track as well. According the track notes on my original album from 1970 the backup singers included Rita Coolidge, Cass Elliot, John Sebastian, and David Crosby. Not bad
Getting back to Stephen and Jimi, there continue to be rumors of an album worth of material from the two of them but I have still not seen any official release. I have a couple of bootlegs that purport to from a recording session between the two of them, I have not way to verify their authenticity.