Maury Ginsberg (appearing as himself) was a technician at the famous Woodstock music festival in New York on Earth in 1969. He hitched a lift to the concert and noticed a disconnected cable that could have ruined the concert. He was not aware that the man who drove him there was the member of the Q Continuum later known as Quinn.
Ginsberg was brought aboard the USS Voyager by Q in 2372 along with Sir Isaac Newton and William Riker to provide evidence of Quinn's influence on history. He was returned shortly afterwards with no memory of this.
Maury Ginsberg (the actor) does not appear to be old enough to have worked at Woodstock, so it appears that for some reason the episode's writers Shawn and Michael Piller decided to use his name for the character after he was cast.