I've found that the posted speeds have little or nothing to do with actual speeds. Our club C rides are slower than posted, B and A are faster than posted. That leaves us C+ riders in quite a lurch. Actually, enough of a lurch that I've considered becoming a ride leader when I am a bit more experienced at such things.

I would say start with the lowest level ride (above the kids rides) and make sure it is advertised no drop. Make sure you have a map and cue sheet, know how to change a flat, and basically won't freak if you are dropped. Then go and see what happens. If they are too slow, all that happends is you've gone on a slow ride. If they are too fast, talk to some of the people on the ride for tips on other slower group rides. In Tucson, there are several clubs and some other "groups" that ride. They range from Cat 1 speed to toodle around the neighborhood speed, so eventually you should be able to find your group.

Our club has information not just about speed posted with rides, but some general skills you should have - like group riding experience, able to ride in a paceline, etc. I've been intimidated by going on paceline rides, but one of my more regular riding partners pointed out that even if I've never ridden in a paceline per se, I've drafted and can hold my line, so I'll be fine. Turns out he was right, because on my last group ride, not only did I ride in a paceline, I actually organized it.