While the letter is published in "The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies", the entire list of signatories is not. It has been published in a couple of other publications, but as far as I can find, the complete list of signatories has not. Being published in the "Official Records" indicates that the National Archives has the original (or a copy of it). That is probably going to be your only source for the entire list.