Socializing 7 month old

I recently adopted a 7 month old terrier mix rescue. Not sure if she was socialized before she was rescued, but her foster mom told me she didn't socialize her. I had a friend over last night and she did not like it. She snarled and barked at them. I had the friend give her a treat, and when we all sat on the couch, she allowed friend to give her belly rubs. Afterwards though, she growled at her as friend was saying goodbye. Anyone have any ideas on how I can make her more comfortable around strangers?