Training a Dog Not to Growl and Snap?

I have an Australian Shepherd who just moved into my house at college with me and 3 roommates. He is sweet and friendly 95% of the time, but I have recently noticed he has some weird quirks. We had a bunch of people over the other night and he sat in a guy's lap who was sitting on the floor, the guy grabbed his front paws to pull him over on his back and my dog snapped at him and made a scary growling noise.

He did this same thing to another person who tried to pick him up. I also noticed he growled at someone who crouched down and approached him slowly like she was going to chase him. It makes me really nervous because I can't have him bite someone at our house. He will not growl or try to bite me. I don't know what to do, any suggestions?

By Lauren R