Check out this blog - it's front range local http://blog.fritesenmayo.com/
It mentions practice on Tuesdays at Belleview Park, and those are open to anyone. Also check out alpha bicycles http://www.alphabicycle.com/ i think the owner mentioned something about doing some adult clinics this year (they have a youth team too) and Alpha also practices at Belleview Park.
I'll have a local race announcement soon!