Say I have a room booked for a party of two but then mid stay another person wants to join and sleep at my room, would that be possible? Do I have to pay extra?
Yes and yes, as long as you are within your occupancy.
Go to the front desk and tell them that you are adding a person to your reservation. They will charge you per night they are there if it puts you over 2 adults - unless you are in a room that allows more. They will give them a MB for the door and front gate, and they will be permitted to use EMH.
Say I have a room booked for a party of two but then mid stay another person wants to join and sleep at my room, would that be possible? Do I have to pay extra?