I've had a really tough time find a group that works for me. I've ridden with one that advertises, "18-20mph," got dropped, caught up because a guy got a flat, then ended up passing some of them on a hill.
Another group said 18-20 and it was unbearably slow. To the point that I told the SAG guy at the back that I'd go off on my own. He tried to talk me into hanging, but I took off. I ended up seeing the group later--after I had ridden a pretty big hill and done an extra 4-5 miles--and was really glad that I split off.

My best, "group rides," have been small rides with friends. Through Strava I've connected with a bunch of locals, and have some triathlete friends. We get together, do a nice ride, all agree on our start time, route, and stops ahead of time. These don't happen as often as I'd like, and I end up riding alone 90% of the time.
Of course when I'm out riding solo I sometimes latch onto other riders or groups. I'll ask if they mind if I join them. We'll usually talk about where we're going, etc. I usually ride in areas that are bicycle-friendly, so it is pretty rare that I'm all alone the whole time for any ride.

For most of my solo rides I hold 18-19mph, but when I do organized tours I'm able to hold 21-22 for 65-70 miles.