It is high, but there shouldn't be any snow, in fact the average High is 67 degrees F for July. That said, the temperature can drop significantly at night (the Average Low is 32 Degrees F) so we recommend bringing a layer or two to keep you cozy.
You're right, there is less oxygen at 11,000 feet than there is at sea level, but if you drink lots of water, and give yourself a day to acclimate, you should be just fine. If for some reason, you won't have much time in the mountains before the wedding, do drink LOTS of water leading up to the event. Colorado is high desert and very dry, which gives you another reason to HYDRATE.
The drive is approximately 100 miles from Denver International Airport to Keystone - HOWEVER - driving through the mountains is a tad different than highway driving. Give yourself plenty of time to make the drive - not to mention you will most likely pull over a few times to take some photos! Google Maps approximates the drive to take over two hours.
If you ladies need help on what to wear on your feet: The ceremony will be outside in the grass The cocktail hour will be on a deck The reception will be inside :) The ground should be pretty solid because Colorado is so dry so sinking into the ground should be less of a concern I would say!