I would think she is a bit insecure about her space. Cats mark their territories, and these "strangers" may be a threat to her. Some cats are just higher strung than others. They aren't different from any animals here, humans included. Some need more space than others, and some are more protective of their living space than others.
Cats have good memories, and it may be that something has happened in the past that she remembers and acting out. Cats can be trained, but not in the same way as dogs. You may want to try to parade a lot of strangers in front of her until she realises it's nothing to worry about, that her food, drink and toys are safe.
Good luck!
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