Five volunteers and 1 cameraman.
From the video description
On July 19, 1957, five men stood at Ground Zero of an atomic test that was being conducted at the Nevada Test Site. This was the test of a 2KT (kiloton) MB-1 nuclear air-to-air rocket launched from an F-89 Scorpion interceptor. The nuclear missile detonated 10,000 ft above their heads.
A reel-to-reel tape recorder was present to record their experience. You can see and hear the men react to the shock wave moments after the detonation.
The placard reading “Ground Zero; Population Five” was made by Colonel Arthur B. “Barney” Oldfield, the Public Information Officer for the Continental Air Defense Command in Colorado Spring who arranged for the volunteers to participate.
The five volunteers were:
Colonel Sidney Bruce
Lt. Colonel Frank P. Ball (technical advisor to the Steve Canyon tv show)
Major Norman “Bodie” Bodinger
Major John Hughes
and George Yoshitake, the cameraman (who wasn’t a volunteer)
see George discuss his work photographing atomic and nuclear explosions in “Atomic Filmmakers.”