"Shoulder" is given to someone when they say or do something really stupid. In order to give shoulder, you must yell out "Shoulder!" Then you slap them on the shoulder. The person who gets it can counter it by saying "self serve", and they are allowed to give themselves shoulder, as hard or soft as they want to. You can also say "no self serve" before they have a chance to say self serve. You can also say "witness", allowing others who see this going on to say "witness", to also give the person shoulder. They can call "no witness", given you have not already called it.
Your friend is trying to push open a door and the u see the door says "pull". You say "Shoulder!" and before you hit them they say "self serve", and hit themselves. Then you yell "Witness!" and all of your other friends hit him on the shoulder.