@Kelly1159 Pretty cheapo of the guests. Up to them to work it out with their friends.
I suppose they might be thinking that they would have booked a smaller, cheaper place had they known their friends weren't coming, but that's not your problem.
If you wanted to be compassionate and offer them say a 10% discount, I would make that contingent on you locking off one or two of the bedrooms, depending on how many they will now need. Otherwise you could give them a discount, only to find they availed themselves of all the beds and bedrooms, giving you just as big a cleaning and laundry job as if there had been a full house.