A bit of a stretch, but maybe the crooked employee tells his friends that he can get them cheap stays at a high end hotel, so they pay him some cash secretly, and he checks them in fraudulently with someone else's card and charges the stay to that card.
That's what I was thinking too and the employee must also let his friends use the name of the previous guests as when I stay in a hotel in Europe they always ask for my ID. Anyways, it's important for the guests that are defrauded to inform the hotel so they can see the pattern.