I just took a listen. IMO, McGuinn's voice is definitely in the mix, but I wouldn't be surprised if they double-tracked it with both McG and Hillman singing lead.
Yes, It's both of them. You can hear it best in the "version one" take on the Notorious Byrd Bros. Legacy edition. As a matter of fact, most of the lead vocals on 'Notorious' are McGuinn & Hillman sing in unison...
I was surprised the song was written by Gerry Goffin and Carol King....another one of their gems....during email exchanges several years ago, the ever friendly and courteous Roger McGuinn (he'll always be 'Jim' to me) told me that he and Gene Clark sang in unison on choruses and even on verses of most of their songs, and Crosby sang the higher harmonies....onstage it had the double track feel of the recording studio. I can see him and Chris Hillman singing that song together....as with the Beatles, it is very hard to single out my favorite Byrd's album from the first years of their reign, but my L.P. of "Notorious Byrd Brothers" is flat wore out.....
"All these things will be lost in time....like....tears....in rain...."----Roy Batty, Bladerunner
I love that song ! My wife loves the version of Dusty Springfield, for my taste a bit thick orchestration in there but wonderfully sung by her; would have been nice to hear her singing on the Byrds arrangement..