Digging up an old thread here, but I just came upon a photo that helps clarify who is in the famous Mac/Dead/Allmans photo above. This one shows just the Dead from what's supposed to be the same evening:
And more of a close-up view of the group jam shot here:
So that's clearly Mickey Hart on the far right of the photo showing members of all three bands. And it's almost certainly Danny Kirwan with his back to the camera, between Bob Weir and Gregg Allman. And that's got to be Duane Allman rather than Phil Lesh right behind Gregg (he's playing a guitar, not a bass), so Phil Lesh doesn't seem to be in the shot.