There are still massive groups all over the Atlanta area. I usually go northwest to the Bud Plant or to Sosebee Bike Park that are within 4 miles of each other. At either place in good weather you may see several triathlon clubs and cycling clubs roll out with many cyclists. As well as, many like myself that will roll out with a handful of close cycling friends (avoid big groups now) or head out solo most of the time. And that is just one side of Atlanta. I can imagine there are other big groups around the metro area.

My best training for fitness is done in a controlled environment indoors in structured setting. With traffic stops and other things I find it harder to accomplish the same level of specific training. The outdoor rides offer long routes and confidence in handling that cannot be achieved inside. An example is a long span of time training inside and then going back to familiar courses and find myself sitting up from aero in T-intersections or tight turns, whereas, when training outside I will blast through those in aero. It only takes a few rides though to get this confidence back.

flame if you want, but I will admit that I don't understand tri groups on tri bikes in large numbers. I've seen 30+ in a tri club roll out. With my fitting I cannot be in a group with my tri bike or else I have to sit up the entire ride. It is a terrible bike to be on in a group setting.

When I go with friends I take my road bike and when the group gets bigger than 10 people it just seems to turn into a mess. My post ride data from those rides are nothing more than crap and they usually turn out to be annoying because of the variety of crazed personalities. Riding with maybe 5 friends +/- is a sweet spot. Some training can be had and good memories too. But I am totally content on a solo 100 mile training ride as well.