Why does my dog growl when greeting other dogs?

Work on her training and socialization to help her. Just do not take any chances and allow her off leash until you are sure she is well socialized. It is probably just a fear reaction but she needs help with that. A fearful dog will bite and you don't want any accidents to happen. Be safe. Take her everywhere you can like pet stores, feed stores, farm stores, outside cafe's or coffee bars. Introduce her to lots of dogs. Have you taken her to a basic obedience class? Very inexpensive and while they do not offer intensive training, they offer basics and lots of meets and greets with other dogs. Remember, a dog should greet by sniffing the butt, not meet nose to nose - not good if you think there are any issues. Bless you and your family for adopting this dog. Good luck.