@Carol-And-Greg0
Unfortunately, it's a bit late if they have already left.
A review is the only way to tell them but it's public, or you could send private feedback if you want to be gentle and give them the benefit of the doubt. You would have a better idea if that was applicable. Possibly, as it was their first experience they didn't think and would not have been so messy had they known.
But, for future in your House Rules you could say guests need to clean up after themselves and leave the room and shared areas clean and tidy.