I wouldn't be as worried about dining reservations, a person missing isn't a big deal outside of the prepaid type events. I was more referring to your room reservation and the interaction with DDP. DDP isn't very flexible in terms of changes/additions and there are minimum notification periods to add or remove it. There are also restrictions on guests registered in the room, and being required to be all on the same plan.
The most common result I've heard is if someone has DDP and they arrive with an additional guest that will be registered, the additional guest is not registered with DDP. That said though, although not a customer centric decision, they could deny registering an additional guest. Other options are being forced to remove DDP all together for all guests or adding DDP for the newly registered guest.