Harpeth bike club has some pretty decent group rides. Their Tuesday night ride is very fast and a great workout. I've found that after going on a few 'published' group rides, people have their own splinter/unpublished group rides that I've invited myself to. hehehe. Williamson County is a haven. If you want to just meet other riders you can do a few of the HBC club rides as a non-member to see if you like it. Also, Pucketts in Leipers Fork is a hugely popular rest stop for cyclists on Sat and Sun mornings.