A platoon guide is a position, but not a rank in the United States Army and Marine Corps.
In an infantry platoon the platoon guide is a non commissioned officer (By TO a sergeant in the US Marine Corps) who acts as an assistant platoon sergeant. The platoon guide is responsible for ensuring the platoon is supplied with ammunition and rations[1] and in combat operations, may be tasked by the platoon commander to take temporary charge of casualties or enemy prisoners of war/detainees until they are evacuated to the rear.
In a basic training unit, the platoon guide is the senior recruit in the platoon chosen by his or her Drill Instructor as a liaison between the Drill Instructor and the platoon and the recruit leadership of the platoon.[2]
The platoon guide also carries the platoon guidon.