A member can call MS or do it online, using the modification of reservation section. For online, go to DVC site, under MY DVC click on My Dashboard, then click on applicable Vacation Plan to open it, then click "View or Modify Details," and then on next page choose Modify for "Guest Information," and then you can make any changes to those assigned to the room. The modification you make there is for room only. Note that the guest cannot be added if doing so would exceed allowed total room occupancy. If you want to do anything like adding a ticket to the guest or a park reservation after adding the guest to the reservation, you need to go to MyDisneyExperience.com and, if not already done, first add the guest as one of your family or friends.
If you are a renter, then you need to contact the broker or the member to have the guest added to the DVC reservation.