+1 - Never heard of UA paying for a rental car and the reason for the cancellation was ATC, having nothing to do with UA. You and your troupe are, of course, entitled to a refund for the segment which was cancelled and which you couldn't use.

However, having received a refund for the airfare, I presume that 15 of you piled into 2-4 vans/cars and actually came out ahead of the deal although the trive kinda rots.