According to the website there's an organized group that leaves from Stone Mountain:

http://www.georgiaridestothecapitol.org/routes.php

Stone Mountain to the Decatur start location
Join BRAG at 9:15am at the gazebo and liberty bell parking lot in downtown Stone Mountain. Departure time for Decatur is 9:30am sharp. Approximately 12 miles at a 12 MPH pace. Contact Vicki at 770-498-5153 or Vicki@brag.org for more information.