Holden Beach is an excellent area for cycling and I usually am there for at least 2 weeks out of the summer. Really organized rides don't happen much there, its usually spur of the moment or planned through the bike shop in Southport. There are the rare organized charity events in Holden (1 or 2 a year). I usually end up riding by myself when I am there, but have ventured up to Wilmington to ride with the bigger group rides up there a couple times (about a 45 minute drive).
Check into the Southport Pedalers(http://www.pedalers.southport.org/) for rides near Holden Beach. Not sure what you are looking for, but they are a definitely a touring, easy ride type group.
If you are looking for more competitive rides/paces faster than 18mph, you'll probably have to head up to Wilmington and hook up with the Cape Fear Cyclist group (http://www.capefearcyclists.org/) or the triathlon/competitive team that rides from the Bike Cycles Shop in Wilmington (http://www.bikecycleshop.com/)