In order to add a guest to any room, there are 3 "gates" that must be cleared:
1 - is there an available bed spot in your stateroom?
2 - is there space available in your assigned lifeboat?
3 - if <18 is there space in the appropriate kids' club?
Presumably #3 doesn't matter for your situation. Even if YES to #1, it could be a NO to #2. It's always better to include someone early and then drop that guest later, because it can be harder to add a guest due to the reasons above. Also the cost could increase the longer you wait.