Summit County (which includes A Basin, Keystone, Copper Mountain and Breckenridge) has a free bus system and is on the way to Vail from Denver/DIA. Will anybody in the group be renting a car and arriving Saturday afternoon or early evening? If so, you can take Colorado Mountain Express (CME) or Summit Express from the airport to any place in Summit on Friday, then hitch a ride to Vail with your friends after skiing Saturday. If none of your friends are renting a car, contact the shuttle companies and see if any will give you a deal on a two stop ride to Vail (where CME certainly goes from DIA and SE might go).
In terms of where to stay in Summit, you can use the Hotels Combined search box at http://www.lodging247.com/.
Staying in Copper, Frisco, Dillon or Silverthorne will make it easiest for your friends to pick you up. Remember that these are all between 8700 and 9700 (with Copper being the highest). If you are concerned about altitude sickness, you could take the Denver City bus from the DIA airport and spend Friday night in downtown Denver, then catch the Greyhound Saturday to Frisco (then have to leave your stuff and catch a bus to the resort, so this will cut into your Saturday ski day and be more of a hassle).
Frisco gives you very easy access to both Copper and Breck with Breck (along with Keystone, A Basin and Beaver Creek) being on the same multi day ticket (or Epic season pass) as your Vail ticekts (which are cheaper to buy in advance online rather than paying window prices for).
Dillon has better access to Keystone and A Basin.
Check the Summit Stage bus schedule/route to make sure that your lodging is close to the bus stop. The absolute easiest location for this (other than staying in resort at Copper which is a very good mountain) is the Snowshoe Motel in Frisco about 10 feet from the bus stop at 6th and Main. Never stayed there, but it looks rather basic.
I ski instruct at both Copper and Vail, so let me know if you have any Ski School (or other) questions.