Hi Chris, 
Glad you're joining us!  Numbers will probably be down tomorrow since there's an MS ride, so group choice may be less.  Average in GSO is around 4200-4300ft/10 mil), minus gravel penalty would make 14-15 at least 19-20, by my guess.  We occasionally have "A+" groups going 21+, then A- groups looking for 20 (although a recent ride went 21), then B at 18-19, then C and D, which would probably be too slow for you.  I can't imagine B would be too difficult, and I expect you'd be fine at A-.  I'll look for you tomorrow (I'll have a solid blue jersey and titanium bike).  If you want to start with a faster group, there's always the option of falling off and waiting for the group behind, or changing groups at the store stop.  
Best,
Scott