Fees associated with adding someone to cabin after booking?
 

Wondering what lines(if any) charge fees to add someone to a stateroom after booking. Specifically booking as a single and later adding a second person.

Also how is the second persons rate determined? If I pay 150% supplement and add someone do they pay what would have been the remaining 50% or are we rebooked at current prices?

TY!

On NCL, my add on paid the difference between my reduced supplement and original rate PP plus taxes. No cancelling and rebooking at current rate.

On Carnival, I was already at full supplement price, so no change in rate. I don't recall if there was a name add on fee.

Carnival just charged me an additional $50 to add someone. They said it was taxes and port? I did not expect this as I had booked solo and had already paid the solo price. However, it is still a very good deal for the cabin location. 5 nights, total price including port and taxes under $800 (Caribbean) for two on deck 10/inside.