This may already have been asked previously but just cant seem to find the relevant post.
We are having problems with Lexi, our 2 yr old GSD, when our children's friends stay overnight.
When people come to the house, knock on the door etc she barks to alert us, which we don't have a problem with. When letting friends and family, that she knows, into the house she calms down very quickly - no jumping up or barking, just affection.
However, recently when our son had a friend to stay over she continued to bark and be aggressive towards him whenever he came into the room, to the point where i had to put her outside everytime he came in.

She didn't jump up or bare her teeth but was barking and stopping him from coming into the room.
I thought it may have had something to do with him being frightened of dogs and she could pick up on it, until our daughter had a friend to stay, who is confident around dogs, and Lexi behaved in exactly the same way.
On this occasion i got the friend to just let Lexi smell her and Lexi was fine, to the point where she was licking her hand, but then again began to bark when she went out and came back into the room.
We are planning a sleep over next weekend and would like some advice as to how to tackle this.
Sorry about the long post.