Many, many people have reported having a package that included the
ddp, arriving at the resort, checking in, and adding an addtl person to the reservation for a day or two later. Those late add-ons didn't have to be on the same pkg as the original guests. In some cases those addtl guests were going to be there for just a few days out of the entire booked reservation...so it made no sense to have them have to have passes and ddp for days they wouldn't be there.
Now, I wish Disney had a way to add those late comers to the reservation, but charge them for the days they would be there. If they were arriving on day 3 of a week long reservation and leaving on day 5, then they could be charged for 3 days, or 2 nights, of the pkg. I realize this won't be a popular thought, but it would make it much easier to add latecomers, or those that would be there for just part of the time.