When you get to the meet up location there are always signs pointing you where to register upon arrival. This is really only for members. If you're just going along for the ride you can skip this and find the food!
There are tons of riders that come along on these rides. A few hundred have been known to show up. (photos were taken at Montgomeryville Cycle - compliment of Walter Kern)
This picture below shows my friend and I waiting to get in line so the ride can start. The group leaders are very well organized. They have a walk-talkie system with varies riders all with in the group. They are probably the most sophisticated "Group Riders" I've ever ridden with.
When you think there aren't any winter riders, go on a Polar Bear Grand Tour ride and you'll change your mind quick. Find a local chapter near you and tag along. It's worth the experience.