I don't know of anything specifically in Buckhead on those dates (Fri 4/13 is going to be dead for group rides) but if you have a car on Sat 4/14 go to either: Atlanta Cycling Vinings ride
http://atlantacycling.com/articles/group-rides-pg37.htm or Dunwoody Cycling Sat. Group Ride
http://www.dunwoodycycling.com/page2.html.
Or check out the Tucker ride which is more of a training/race pace: "Tucker Ride-Strenuous/Training. Meets at 9 am during DST and 10 am during Standard Time at Tucker High School on Lavista Road. Show what you are made of! Rides from Tucker to Stone Mountain and back. VERY fast paced training ride, 24 mph++. Terrain is hilly, and distance is about 43 miles.*This is where the big boys and racers show up to play."