Even if there's a chance someone will be joining you, and if you have the guest capacity on the reservation, I'd add them. There's no negative to having them on the reservation that I can think of. Something you might consider if you do add an additional guest - double check in your
MDE to verify the additional person has been recognized on your reservation. They should come over to the MDE link from the
DVC reservation when you add them, but check. I believe it's the reservation and guest list in MDE that cast members at your resort will see.