For those interested in group riding, I'd suggest finding a small, local group to ride with (maybe through a bike shop or local club) versus charity rides as a good way to get used to riding in groups. Charity rides, IMO, bring out some not so great behaviors, both with new riders and experienced riders alike. In fact, the experienced riders zipping by in fast pacelines can be the biggest hazard in my experience. Ride predictably and to the ride of the lane, and keep an eye out in all directions, including what's going on behind you.