I haven't ridden with them, but have been thinking about Rocky Mountain Cycling Club (www.rmccrides.com). Their schedule looks pretty interesting in terms of different locations and building throughout the year.
another one you might try is Heartcycle Colorado. I believe their site is heartcycle.org I didn't do any rides with them this year but I have in the past and found them to be a nice group...all abilities. Rides are troughout the front range including some rides in the south part of town