The easiest is to add them to the entire stay at BCV. You are entitled to add up to 5 adults to a BCV studio and doing so allows access to SAB which I assume is the main drive. DVC not Disney is going to care if they don’t stay overnight.Can they be added to the reservation on site? No dining plan involved, it would be the last day of our stay. This would make 4 in the studio.
It is much more seamless to simply add them to the room in advance, rather than at the FD (especially if the FD designates them as a "day guest" - lesson learned the hard way )