I actually just moved to McDonough a over the summer as well. I live right at 20 and I-75, so i've been going up Willow Lane and shooting out Mt. Carmel to access to nice roads out near Hampton and Atlanta Motor Speedway. I like riding down (and up) Wynn Drive in Hampton-- it's a pretty nice hill (for McD) with virtually no traffic. Also, if you're looking for a longer ride, take Upper Woolsey Rd. back behind AMS. There are all sorts of open roads out there. But I use Carmel for access, because I got yelled at one too many times by natives for trying to ride on 20 (how dare me).
For a BRUTAL route, take Rocky Creek Road to Griffin. It is hilly the whole way, with a couple of extremely brutal hills right in the middle. However, I think there is currently construction at Rocky Creek and Hampton Locust Grove Rd., so I've been sticking to the northern route.
I might have seen you riding around. I've just seen a few people... I ride alone. I did the group ride at the Trek store a few times, but group rides seem kind of dangerous to me, so I stopped doing it.