During the phase where a guest completes their reservation please have them list info for EVERY guest. So if they have made a reservation for 6 people, they would be provided with 5 additional fields to list the names (I'd love to have age as well, but I'd settle for a name).
This would also be an ideal place to add a note about this host does or does not allow additional unregistered guests on property
There could also be a note here that says "notify your host if this guest list needs to change"