Here are some links to groups that do rides together in the Front Range area. The third one isn't woman specific but the first two are and they all seem pretty active. There are all different skill and fitness levels in the groups so anyone can ride with them.
*****in' Mountain Women http://www.bmwbikers.com/
Singletrack Sistahs http://www.singletracksistahs.com/
Colorado Mtn Bikers http://groups.yahoo.com/group/comtnbikers/
HAVE FUN!!