Not much help, but I miss Alpharetta so much!!! Used to show at Wills Park all the time as a kid...are they still doing lots of shows there? I'm sure a lot has changed around that area-loved going to the tack stores too!!!
There is not much for strictly eventing barn in Alpharetta/Cumming. (GA-400 corridor). Check w/Area III website, local ad rag (Stable Mates). I'm at a H/J barn, next door to GP dressage trainer -- have to haul off for gallops/cc -- but I'm only doing Training at max for right now, so it works.
Sadly Stablemates is gone. UCF has a galloping track a few CC jumps and trails. Wilson farms has regular shows a nice arena with mirrors and some nice CC jumps as well. Both places have a nice group of people.
This is not an eventing barn as far as I know (h-j I think) but it is near Alpharetta/Cumming, has been around as long as I have (they used to show at Atlanta Pony Club shows when I was a child) and still run by the woman who rode for the barn back then. They must be doing something right!