We created a cycling club here at work --got work to subsidize jerseys and entry fees for organized rides and races. We have folks of all levels, but really there's only a core group of maybe ten of us (out of 45+ on the distro list) who ride/communicate with any regularity.

Anyway, I put out on the group calendar a 2x/week "fast" lunch ride. Now for the talent here at work (other than one Cat2 I know of), "fast" means low20's mph pace, sub-20mph average. Even still, I'm solo most of those days unless a couple (literally) of the other guys join. So defining the ride with strong language may mean you don't have any takers. Also, keep in mind that most of us in the "serious" category that you're talking about probably already have their group rides.

One way to cultivate a new group is to rally around an organized ride. When I lived in Virginia I hooked up via word of mouth with some folks getting together to initiate an organized ride (http://www.fanfreeclinic.org/events/ride-to-provide/) so we all started training together. But the time the ride came and went, we were far beyond "recreational" and enjoyed riding together at higher speeds and tighter pacelines....