Originally Posted by FozBoy

It is most certainly not a criminal offense: not reporting one's guest identity to the questura (not the carabinieri…) is a very minor administrative offense punishable with a fine (of course, in the event the unreported guest is not a fugitive from justice or an illegal immigrant, in which case failure could be a serious criminal offense).

The municipal tourist taxes are levied on the hotel, not the guests, so by not declaring a guest one is not committing tax evasion. Just because hotels pass-on cost of the tax to consumers it doesn't mean that guests are liable to pay for it.

And, if you do decide not to report your guest and get caught by the hotel staff, I would not expect any carabinieri being called. Front Desk will just ask for your friend's passport details and charge the extra guest fee to your room folio, unless you're exceeding room capacity, in which case they will ask him to leave.