The Ski Train is dead. I know - I worked on it some this past season. It may or maynot get revived this coming season. It only ran on weekends. It only ran to Winter Park.
There is the CU Ski Bus for students.
Commercial buses run to resorts from the Dinosaur Lots in Golden to parts everywhere.
Boulder Craigslist - I haven't run into a serial killer yet.
SkiCarpool.com - I've used it alot and always had a great experience.
Boulder Snowboarding Group -
BOULDER SNOWBOARDING GROUP (Boulder, CO) - Meetup.com
TGR is where you'll find your dedicated riders. Plenty live in Boulder. I live just outside Boulder.