This is to answer Grog's question. My rides are social because while some of us are close together, others are riding side by side IF we are on a rural country road, and others are passing and/or moving back to talk to friends.
We also stop and regroup. AMC requires there is a sweep. That is usually me when I lead with my husband, so let's say at the top of a climb, the group will wait until I show up behind the slowest person. We don't drop anyone. MOST of the time all of the riders are within a speed range, but last year, on all of our rides, we had at least one "outlier." It was torture for me, mostly because each of these persons misrepresented their skills when I screened them. Now I am so fricken to the point when I screen riders, they truly understand if it's the ride for them. Every single time the person has been a woman, who lives near the city, who is not used to the smallest rollers and hills. So they say they can ride 25 or 35 miles, but they lie about their average and the hill thing. These rides are not that fast to begin with, and there are "easier" group rides in the club, so there is no reason to lie.
We stopped leading the show and go rides, because there are too many unknowns. After 3 years, I know most of the people who sign up for our rides and the new ones usually are nice people who have found the type of ride they want.
The other club I ride with is part of a more traditional bike club. However, it's a group for "older" riders who can ride during the week. I hooked up with them in the summer when one of them came on one of my rides, that I lead alone on a 95 degree day, when my husband was in the hospital. Of course, there are a lot of younger people, 30's-40's who come, too. These groups tend to be huge and some are very squirrelly. I stay up front and that avoids the issue. Many of these people are great athletes, have run the Boston Marathon many times and ride long distances. They are just older and slower now.