There are a number of great clubs in the ATL area. Covert Hops, Alpharetta BrewMasters, South Atlanta Brewers, Final Gravity, Chicken City Ale Raisers (Gainesville), et. al.
Being in Decatur, your most convenient group is the Covert Hops Society(http://groups.yahoo.com/group/coverthops/message/2604
They are well organized and seem to do the most group things together. Monthly meetings vary from Twains, to Brick Store, to other downtown and Decatur establishments. They hold quarterly group brews on a communal brew system. And they hold the largest comp in GA. I hope to come up in Feb/Mar this year to judge at Peach State.
Final Gravity also meets in Decatur often and are a bunch of good guys. http://finalgravitybrewing.com/drupal/
The Brewmasters probably have the best collective number of brewers (not trying to piss off any of the other clubs but its a fact!) as they tend to win a lot of competitions and have the current AHA Cider Maker of the Year in their fold. Or maybe they are just more into competitions (I just like to drink their beers).http://www.brewmastersopen.com/
Chicken City Ale Raisers have the highest qualified BJCP judges in the state with 2 Master Level. Phil Farrell is big into teaching new BJCP classes each year and holds at least one exam for ATL-area brewers.http://www.ccarhomebrewclub.com/
So there is a lot to choose from and you won't go wrong with any of them. Try out a meeting with each and see who you feel most comfortable with.http://sabrewers.net/