What happens when two people stay in a hotel room reserved for one?

For security reasons it's forbited if they was ever a emergency such as fire the hotel will have to accounted for everyone to be out of the hotel. Plus the Rate of the room will be different of course. We need immediatly to speak with the guest in a gentle manners and explain that we have other rates for2 person ( if applicable or not ) but it's necessary to register the second person in the hotel.

Depends on the hotel policy. Usually you get charge for extra head  since it is a room for one. Except if you're VIP then you'd be allowed to bring people free of charge. 

Whatever happens depends on the rules and policy guiding the accommodation of guests in the hotel.

So if defend the situation there's no problem any staff or receptionist in the hotel even two people come in reserve for one maybe the people don't budget for reserve two so that's why we will choose the cheapest price room in the hotel so this is the happen sometimes understand the guest or show some care thanks

Be professional and manners talk two people reserve in one people only..

reserved room double or single?        

are the two guests have any kind of relation family, couple, friends.....?     

if the room is double and, they have relation no problem at all. the one who reserve the room will be in charge. 

I approach them nicely and explain the policy, ask them if  they want another room for couples.

Major compromise takes place effecting the quality of service

In the place of security reasons,We will need immediatly to speak with the guest in a gentle manners and explain that we have other rates for 2 person but it's necessary to register the second person in the hotel.

It appears in the guest list as an increase, the reservation must be modified to a double room

That are depends on the rules and policy of the hotel.