I'd stick with the ignoring route for now, and get her in a puppy class with a trainer that has mastiff experience. If you have a crate and can keep her in the room with the strangers, but contained in her "safe den", that might help for now, too.
She might be just voicing her uncertainty with the situation, and ignoring her will let her come to her own terms with it. Getting her in a class and/or working with a trainer will give you some specific ideas on how to help her.
You'll want to build her self-confidence, so coddling or "saving" her from the strangers would be counter-productive. Try and expose her to new stuff that she can handle (boxes, balls, trees, flags, etc.), and slowly build up to bigger items (like people). You could try putting a command to it, like "go check it out"... and have treats hidden in the boxes or behind the trees for her to find. Turn exploration into a game for her.
Be patient, calm and don't worry about it. She's still young and trying to figure things out.
I haven't had a puppy that young that would growl or bark, either, so hopefully some more experienced mastiff people will chime in with some recommendations here, too.
