John Mayer will make a guest appearance during the Zac Brown Band’s Southern Ground Music & Food taking place Friday and Saturday (Sept. 21-22) at Nashville’s Riverfront Park. Mayer, who underwent vocal cord surgery in August, will not be singing but will play guitar both nights. Mayer and Zac Brown Band member Clay Cook met in Boston in the mid ’90s while attending the Berklee College of Music and formed a duo, Lo-Fi Masters, before moving to Atlanta in 1998. Gregg Allman and Sheryl Crow will also be making guest appearances on both days of the Southern Ground festival. Additionally, Alan Jackson will be a guest on Friday, and Dwight Yoakam will sit in on Saturday. The festival also features sets by Amos Lee, The Lumineers, Blackberry Smoke, Los Lonely Boys, the Jerry Douglas Band, Nic Cowan, Sonia Leigh, Edward Sharpe & the Magnetic Zeros and Michael Franti & Spearhead, among others.