I ride with one group that are fairly slow recreational riders but alot of fun to hang around with. The same club also has a much faster group that rides on a different night. I've started riding with that group as well,and have found enough middle-of-the-road riders to join in so that we can just split up into mini groups and all still get the workout that we're looking for. The fastest riders in the group tend to average around 22mph, but now there are just as many who stick around 17-18. Some of these people had been scared off previously but now are enjoying showing up since they won't be left on their own.

Maybe you can get a few of the faster B- riders to join you in the B-B+ group and you'll all get more of a challenge.