I've had two different experiences with Group Rides.
The first group I was ever part of was a local group in NJ. Very polite group of people - 30 - 40 rides each ride, all split into 3 groups (the 'A' Group, the 'B' Group, the 'C' Group). Ride leaders for each, organizing the groups (met in a parking lot of a National Park or some such). The groups headed out in turn - A first as they were the fastest, etc. C group for the casual riders. I was about 10 - 15 years younger then anyone at the time (senior year of HS for me) but everyone were friendly. I tried a few rides with the 'A' group - I could hold my own, if not lead most of the way, in the rolling Hills. But the Great Swamp (which, compared to FL, is not swampy at all) was a pancake flat stretch of road 5 miles long. I would get toasted by the A group there. So I joined the B group, and rode at the front with a bunch of older guys the whole time. We'd get back to the starting point and they'd invite me to do a few extra miles, which I was fine with - especially if my dad was on the ride as he'd be with the C group who had yet to come in). Worked out great. Single file on the busy roads, always moving out of the way for cars to pass etc.
The second group, which I only witnessed from afar and never joined (for good reason) was in Florida several years later. They'd meet at the local school and head off from there. I was riding the opposite direction. A cyclist came up and passed me, so I started drifting off of him. Always nice to find some one to ride a stretch with. Up ahead, a huge group of cyclists were taking up the entire road, cars trailing behind them unable to pass. The guy I was behind was joining up with them, slowed down and blocked my way around him as he hugged the shoulder getting ready to U-turn in the road to join his compatriots. I was unhappy, having to ride my brakes. He thought I was part of the same group. The group seemed very elitist and were riding dangerously. I saw them on a few other occasions and none of them ever moved over into single file to let cars pass. I preferred to ride on my own.
Now that I am in CO, I rarely see a group of more then 5 riders. Most people seem to go solo or with one or two friends. All move over to let cars pass and generally ride safely. The average joes are the idiots out here - was riding the bus today and saw a guy in t-shirt and shorts (and headphones) riding head on into traffic, on the wrong side of the ride (aka heading at the bus). People like that frustrate me greatly. Its one of the dumbest things you can do. A cyclist going the right way had to pull over to let the moron go and I think told him off (the moron shook his head about something). Oddly, there are tons of bike paths out here, especially leading into town traffic free, with one street being a Pedestrian-Cyclist Highway (or some such) where cars are only allowed on it for a block at a time. Why not use that instead of riding the wrong way in heavy traffic?
Different group dynamics depending on where you are in the country it seems. I ride solo out here myself. Don't really know anyone and there don't really seem to be established bike clubs (the only out here I know of is all about racing and training for races or doing TT's which is not for me) out here, just random groups of people hitting up the various Mtns.