German Shepard biting guests?

We adopted a 4 year old neutered German Shepard about 3 months ago...he barely even barked when we first got him and people could walk in and out without him barking he would greet everyone wagging his tail...then about 2 month of living here I think he realized this was his new home and he started barking when he heard people at the door and ever time a door opens he runs to it barking with his hair up....a week ago a friend came in with a hat on and a puffy winter jacket he was fine but when the guest got close to me my dog nipped him not hard and today a different friend walked in with a puffy jacket with the his hood up and a hat on my dog ran at him and he turned quickly when the dog approached him and he bit him hard broke the skin through a shirt a sweatshirt and a big jacket then was fine with my friend until he went to leave and put his hat and coat back on he snapped at him but didnt get him I dont know what to do I like the idea of my dog attacking a unwanted intruder but im worried about friends being here with him and also if he would turn on us one day? I also have a cat and am worried if this aggression progresses my kitty might be in danger any advice would be helpful