Just want to say one more thing to clarify.... The problem with punishing a dog when it is fearfully growling, is it does not solve the underlying problem of being afraid. You may be able to teach the dog not to growl, but underneath, she is still scared.
Growling is a dog's way of communicating that she is getting nervous. Its a sort of warning. If we take away a dog's ability to warn us, then the next behavior (such as a snap) can come as a total surprise.
In general dogs do not want to be aggressive. They give us a whole series of behaviors to tell us that they are uncomfortable, fearful, very fearful, panicked, and finally they feel the need to lash out. Your pup is certainly giving you some body language before she starts to growl, that indicates her discomfort with the situation. Watch her carefully. Is she trying to turn away from people? Does she lick her lips? Is she acting shy? She is telling you the "pressure" of people so close is bothering her.
Ideally, we listen to the pup before she feels the need to growl and modify the situation so she is not feeling so threatened. For example, have people sit on the floor, and let pup approach at her own comfort level.
In the long run, you want a dog who is confident and comfortable with people. The way you create this is through many pleasant experiences with new people.
You model being relaxed with others, and she will follow your lead. Just let her take things at her own pace. She's a tiny little thing and her world is full of large, potentially scary people, who do not understand "dog language" so they are often displaying behaviors that are very threatening to a dog: such as getting in her face or controlling her with their hands.
Anyway, the book by Turid Rugaas would make all this very clear. She includes lots of pictures. I got a lot out of it and recommend it to anyone with a dog who displays some nervous behaviors. Good luck!