Grand Lake.
We have good snow for early December. You can legally ride your sled in town on the streets and to the trails, (although I'm not sure the town trail is open yet). 2 hour drive from Denver (normally). Very well groomed trails with plenty of off-trail ridding. Big rental fleet if you need a machine. Polaris dealer right outside of town, Skidoo/Artic Cat Dealer in Granby (15 miles). Good map with plenty of on-Trail Markers.
Look up Grand Lake Chamber of Commerce for places to stay, but if you have a larger group, we have lots of second home owners who rent nice houses, some right on the trail or easy access to it.
Snowest Magazine just picked Grand Lake as the best place to ride in Colorado.
If you do end up in Grand Lake, look me up at The Rapids Lodge/Restaurant, I'll buy you a Shirley Temple and maybe we can swap some riding stories. Max