I think most rides will average less than the published averages, as those averages are published to warn people about how fast the pace could be, but rarely is due to traffic lights and terrain. My regular A ride (a race team, with other racer-types joining in) typically averages around 19 - sometimes pushing 20. But that ride includes a fair amount of climbing, and some fast pace work that will leave anyone who is not race fit gasping and/or dropped. While the ride you mentioned may publish a higher average than we typically ride, most of the riders from that ride (and as you know I've been on those) would not be able to keep up with our team rides.

Even Around The Mountain (not the toughest, but certainly the fastest ride in the Valley) will average under 20mph, but that will include climbing South Mountain, and the fast section will average just under 30mph, on the flats, for over an hour. The rides I've been on where it is hardest to stay with the group are probably 'BOS', the Bicycle Ranch A group, our Faster A group, and from what I hear, Focus Cyclery. But none of those will have a particularly high average speed - the difficulty is how fast they take the climbs.

The only way to know what a group ride is like is to give it a go, preferably working your way up from a sub-group you know you can hold, to the faster sub-group. For instance, I did Bicycle Ranch B a couple of times before I tried the A group. Now I ride with Faster, and the only time I ride the B group is if I'm doing base and have coaching instructions to avoid high-intensity.