Super shuttle is uber sucky. Not the way to go.
I would recommend having people take RTD to the stop in Broomfield, and then having an independent shuttle from there. (Have a person rent a van for a day and have the people call when the bus drops them off).
Alternatively, you could just do the rental van thing doing runs straight from the airport. You just need one person with the spare time.
I think that might happen--I might rent a mini-van, and make a few trips to and from DIA. I pondered the RTD option, which I use all the time when I go to downtown Boulder, but the Broomfield P&R lot is pretty far from the hotel (a bit too far to ask the hotel shuttle to pick 'em up, which they would if the P&R was a bit closer).
I think we'll go with a combination of Supershuttle and a couple of minivans. Thanks all for your advice!