I've flown in and out of DEN in FC many a time, and I sure do not know of any FC lounge run by UA available for use by domestic travelers (assume you are flying domestic?). Aren't those lounges restricted to primary overseas departure airports? Believe me, if there were such a lounge available for domestic use, I sure wouldn't bother with the marginal RCCs there. Shoot, if they can't even afford to maintain a 1K room at DEN, guess a FC lounge would be out of the question.