The group I've been riding with goes out of the Atlanta Cycling shop in Vinings. They have rides on Mon night, Wed night and Sat morning. They also run the Buckhead Bellyache ride out of Buckhead on Thurs nights. You can get the info on these rides at Atlanta Cycling .

There's always a few hard core roadies who will look down on someone with a mtb on a road ride but my feeling is who cares what they think? We're out there for some fun and exercise, not to win races. I would think the main challenge would be to keep up with the group (the routes are well-marked so it's not a big deal to get dropped though). Like you said, the mtb is naturally slower on the road than a road bike.