Two years ago, my former TA forgot to put my FIL on our reservation. Her advice was not to worry, don't tell them, they'll never know. I couldn't do that, and told them at check-in we'd have a third adult. We checked in three times that vacation (we like to switch resorts) and we were never charged for him. Each resort told us that he's a senior citizen, we won't charge you. This way, however, he had his own room key so that he could come and go on his own, and could charge back to the room if he wanted to. Fortunately, my TA had put his name on our cruise reservation!
Beth