We organise free of charge shuttles from Aspen airport. There is no need to reserve in advance for these.
From Denver there are buses running every second hour for around $120 each way, http://www.coloradomountainexpress.com/.
Another option from Denver is to rent a car, which can often work out more economical (depending on your group size). Parking is available in Snowmass from $10 per day.