I've booked rooms alone, and then added friends on when we made arrangements. You will want to add your friend on when it is confirmed they are coming, though, because you may get asked for cards at the pool, etc...
Are you asking about a package, though? I've never booked a package, so I can't be sure, but I think everyone staying in the room has to be in on the package... But, you would still be able to add them later, you would also just add their pass at the same time...