people stay all the time! But like its been said, thin thin thin walls and ceilings, I dont want to hear anyone have sex(and havent) and i dont want people to hear me having sex (which i pray to god they havent)
If you're rule conscious though, you just submit their name and yours to your reception to say they're staying, and they say its fine, but you're meant to give 48hrs notice, but if you're talking about randoms, then no one really cares.