Denver airport (DEN) is the easiest and most direct option to get close to Keystone if you plan to fly.
Southwest airlines typically has great rates to Denver!!! Be sure to check online as they usually are running web specials!
Frontier also usually has similar rates to Denver!
Getting from the Airport to Keystone, CO
This is about 95 miles (about and ~1.5 hour drive). You have a couple options and you will not need a car to get to the wedding activities if you are staying at any of the recommended condos or hotels.
You can rent a car
You can also take the Colorado Mountain Express shuttle.