There are largish group rides out of Dunwoody, which is just a little southwest of John's Creek. You can see a full schedule at the link below. The rides are generally A/B split according to ability and usually feature a pretty good-sized group, even during the winter. It's generally pretty heavily masters focused but there are occasionally racer-types that show up. Both Saturday and Sunday courses are interesting and feature a few short (<2 minute) but somewhat steep climbs.
If you want to head a little east instead, there are rides from Suwanee as well. These rides are usually a little smaller than the Dunwoody group and they often have younger, more race focused riders but are not what I would call "race rides." The terrain in Suwanee is a little different, generally no steep climbs and the roads are usually lighter trafficked and have a wider lane width.
Personally, I would recommend you do the Dunwoody AM ride on Saturday and then the Outlaw Ride on Sunday. This will give you a really great view of the area and should be a fun experience. Please let me know if you have any questions of would like route suggestions getting to/from any of the rides.
Here's a comprehensive listing from Sadlebred. As mentioned above, the listing includes a lot of rides that don't run this time of year and also includes some rides that no longer exist so you may need to do some due diligence to make sure the rides are going off.
Southeastern Cycling | Rides