Help with Territorial Dog?

I have an australian shepherd female who is about a year and a half. Today I was unloading the car with her and our other dog, when the neighbors came out of the house next to us. She ran up to them and barked at them and scared their little boy. They were pretty upset, with good reason and I feel terrible about it. Her behavior shocked me. I've never seen her do this before. She will bark if someone knocks at the door, but all dogs do that. She will also bark at people from the backyard. She has a big mean bark, but she is in no way a mean dog. When someone comes to the house, she's very friendly and is fine with everyone. The only barking I've seen at people is when she gets startled but she responds fairly quickly to a command to be quiet. I'm pretty sure she's being territorial and I really don't want her behavior to get worse. What are some training options for this? I've already decided to not let her on the furniture anymore. Is there any exercises I can do with her? I really need some good advice for this. Thank you.