I just called to check on this, and as long as you are not adding a room and you are not at room capacity, you can add extra people. They allow only two adults in a room at the rate quoted so if the addition will exceed that amount you need to pay an extra $10/night. Like any package, an additions need to have the same number of park days as you, etc. The CM I talked with said that it can be done anytime before your trip without a fee because it is considered an upgrade. He defined upgrade as spending more money.
I also asked the CM about my original question (everyone present at check in) and he said it was not necessary. I hope he's right!!!