Andrea,
Sorella Cycling (www.sorellacycling.com) has a beginner's/recovery ride on Monday nights out of Avondale, near enough to Decatur. It begins at 6:30 from the parking lot behind Avondale Pizza, but check the website because the times will start moving up as it gets darker earlier. Please make sure to tell the ride leaders that you're new to the ride AND new to cycling. They'll make sure you don't get dropped, and someone will fill you in on the rules of the road. Helmets and positive attitudes required.
Other places to look for rides is www.bikesbl.org -- the Southern Bike League has a calendar of rides all over town.