scared, shy of strangers 3 month old puppy

Question
I just got a female chow/golden/shephard mix from the pound. She was only there for 3 days.  She was spayed on Monday  Is housebroken!  Does not whine when confined (I do not crate her but she has her own "sleeping room".  Friendly and affectionate to both me and my husband but dedicated to me already.  
When she sees other people she barks and growls and backs up.  Other dogs she is fine. People with dogs no problem.
What do I do to help her get over this fear of people.  I am afraid that she might have been hurt when she was very young?
Other than this she is responsive, intelligent and energetic.  
My instinct is to let her be and let her warm up to the persons on her own.  But I don't like the growling.  I do want her to bark at strangers when they come to the house so how do I show her the difference.
No other issues at all.  Very very smart dog!


Answer
It may be less of a problem of being abused, and more just a lack of being exposed to strangers.  One of the best things to do is to show how happy you are to see the people.  She will take her clue from you.  If you tense up fearing her reaction, she will sense it and interpret it as the stranger causing the problem.  Having visitors offer her a treat will help too.  

Thank you for adopting her.  We have so many dogs needing a good home.  I hope otherwise things go very well.