Sgt...
Last time I was in Colorado on my CB it was 45F at noon in Leadville (10,000ft) on a sunny day, and I didnt have enough warm gear. I beat it back to I-25 and it was 87F, same day.
I'd advise your pals to do a little riding before the trip to polish/refresh their skills, do you have MSF or equivalent there. It is easier to stay out of trouble, and get out of trouble on the smaller bikes. Or maybe you could do a two looper, say to Estes Park, Peak to Peak highway on the smaller bikes, then return to Denver and swap for bigger bikes and head for Durango. Just an off the wall idea.
Sounds like a fun trip!