One of the clubs I belong to has 5 groups. The As are advertised at 23+, the fastest Bs are 21+, the middle B's are 18-19, the slowest Bs are 16-17 and Cs are slower than that. That, as I said, is what they advertise. I ride in the slowest B group and my averages are consistently in the 18-19 range when I finish mid-pack, so they lie a little.

Keep in mind these sorts of ratings will be all over the map depending on where you are: the local riding culture, the members of the club, even the terrain.