A man was shot and injured at a large party Friday night in North Plainfield, the Somerset County Prosecutor’s Office said. The 20-year-old Plainfield resident, who was not identified, was shot after attending a party on Arnold Ave, officials said. It’s unclear what prompted the shooting, and no arrests have been made. First responders were called to the area near Vermeule Park around 10:45 p.m., where the man was initially treated for gunshot wounds to his upper body, police said. The man was then taken to a nearby hospital for non-life threatening injuries. “Authorities are attempting to det…