Georgia is, at lkeast to me, one of the most confusing of all the onfederate states with respect to its "home guards. Every adult male was required to enroll in the militia. There is a ground-breaking compilation of the 1864 militia enrollments by Militia district, at least for most of them. I can't recall the compiler just now, but if you have access to a good sized library in Georgia, ask for the 1864 Georgia Militia enrollment book. These men were not necessarily called up -- apparnelty in msot cases they were not, but it gives an idea of who was still at home in 1864 -- if they bothered to enroll at all. There were indeed active militia units, called to service; some of these are shown in the Compiled Service Records, but in many cases they were not.
In addition, Georgia enlisted a number of reserve units, composed of young boys -- age 16 and int hey went -- or the older men -- by 1864 that would ahve included the men between 45 and 55. For instance, it was primarily reserves that were used as guards at Camp Sumter (Andersonville), and they died in appalling numbers too.
The Confederate section in the Americus Cemetery nearby is full of these reserves. (I believe a few companies of the 55th GA that escaped capture at Cumberland Gap were also guards there too).
Other classes of "home guards" were the Georgia State Troops and the Georgia State Guards. There were numbers of these units, which were essentially militia, called up for specific periods, usually six months.
The there were Joe Brown's Pets, two infantry regiments with a few independendent cavalry companies that were raised for home service by Governor Brown, with the implicit promise that they were do bombproof service in Georgia. But when Georgia was invaded in the spring of 1864, they were called into active service, and did honorable service in the Atlanta campaign. There is again a book -- the name doesn't come to me on the spot -- which details the service of these units, and gives a roster, which is helpful, since I don't believe they managed to creep in to the Compiled Service Records either. The companies were mainly from North and Central Georgia.
Outside of all of this were the Arsenal cadets, who were cadets at the Georgia Military Academy at Milledgeville, called into acvtive service in 1864.