@Tamara404 Your "Host Guarantee" is effectively null if there are any unregistered guests on the property, so if you choose to permit an additional person to be present - even if not overnight - you should require that the guest agree to amend the guest count in the booking.
If your listing normally charges an Extra Guest Fee above 1 guest, then I see no reason not to apply it here.
But I'd also look upon this request with some suspicion. If the "friend" is living in the city anyway, why can't they stay in their own home?