I have copies of this photo in three books, and the clearest with the least cropping is in "The Best of Life" (Life Magazine), Avon Books, New York, 1973, page 41.
Unfortunately all three are cropped at, or a little to the right of, Ruby's left elbow, so the man concerned isn't visible.
On page 218 of the Warren Report there is a photo from a TV camera's view of the shooting, Commission Exhibit No. 2636. It shows part one of the Ford Galaxies and is taken from about 90 degrees to the right of Bob Jackson's position. The only identifiable individuals from Jackson's photo are Ruby and Detective James R. Leavealle in the light-coloured suit and hat, to whom Oswald was handcuffed.
However, there is someone in a dark jacket reaching out with his left hand, but he is much closer to the TV camera than Ruby, Leavelle and Detective L. C. Graves. There is also a hint of another arm just above the left one showing, but due to the limited resolution of the TV image it's not possible to tell whose it is or whether it is, in fact, an arm. If the entire TV clip is available, viewing it might help.
Do you know of any online links which show well-detailed and uncropped copies of Jackson's and Beers' photos? In my copies of Jackson's, the only hand visible beyond Ruby's hands is a right hand holding Oswald's upper left arm, so most likely that of Graves.
Changing the subject a little, I picked up my copy of the Warren Report for $1 in 1995 at a sale by my local library. On comparing it with the online version http://www.archives.gov/research_room/jfk/index.html
about 2002-2003 I was horrified at the large number of errors in the scanned copy. Simple scanning errors that hadn't been corrected. I offered to help correct a few hundred, but never heard back from the archives people and haven't checked since to see if corrections have been made.