Dogs are pack animals, and in the absence of other dogs, your family becomes it’s pack, to be defended and cared for as a priority.
Ridgebacks tend to be loyal, and will always bark when they spot someone on their home turf (for example, someone walking to your front door from the road). They have excellent hearing and sense of smell, and will often alert you to visitors with loud barking that will not stop until the visitor leaves, or is introduced to the dog.
However, if it’s a member of their pack approaching (for example, a family member coming home after work or school), the barking will settle down to whining – they want to be reunited with their pack promptly, and will be very pleased.
Regular visitors tend to be considered part of the pack after several visits.