I have to agree with Irulan. Lead a slower ride some days and go hammer with the A group other days knowing you may be dropped. Good training means you mix up the pace anyway. I've gone out with my two co-workers a couple of times, took my town bike, rode slow and talked about bike handling, etc. It has really been rewarding for me. I get slow recovery days but more importantly its making me think about what I went through in the early days. I'm finding new inspiration from their joy of discovery. I also do little things on the bike path with them that I don't take time to do when I'm on harder rides - practice riding without hands (can do on my mtbike but just getting up the guts to do it on the road), countersteering even the smallest of corners, turning waaay around to look behind me, slow races, etc. Little things that get passed up when you're always out hammering.

I have to think that if the word goes out that you are leading these B rides that your credibility within your local bike community will go up and you may attract riders in that mid-range speed that you're looking for. Maybe even end up with decent size A, B and C groups.