Rottweilers are intelligent enough to have done a number of jobs throughout their history. They originally served as livestock herders and guardians, as well as pulling the carts laden with meat. When this job was made largely obsolete, rottweilers became popular as police dogs, search and rescue dogs, and personal guard dogs.
The same devoted nature that makes your Rottweiler adoring and patient with your family makes her protective of them. Your Rottie has a natural instinct to guard you, your family, and your property, so training your Rottweiler for protection is about teaching her the right times and circumstances for guarding, and how you want her to guard.
If, however, your Rottweiler is overly aggressive in the wrong situations, like continuing to growl when you have greeted and invited in guests, you will need to curb your Rottweiler's instincts and give her outlets for excessive energy.
Rottweilers who are happy and friendly with strangers, even at the house, can still learn to protect. Young dogs, especially, may not have the instinct firmly developed yet but will develop it with time and training.
If she thinks that she must decide what to do to protect you she will have an anxiety-laden life and may guard inappropriately, resulting in a bite to a friend or service person. Instead, teach your Rottie what situations are appropriate for guarding behavior, and what you want her to do to guard you, your family, and your property.