A friend of John Hammond in the game Jurassic Park: Trespasser. Harold probably went to Isla Sorna with an 'electronic device' after the Isla Nublar Incident, but before the events of The Lost World. In his memoirs, Hammond describes their first meeting:

"I first met Harold Greenwood in 1992, 
he was a, an American. Introduced to me as 
a former Green Beret. He asked a number of 
questions about the disposition of the InGen 
Technology. Harry claimed to be a friend of 
my former son-in-law, I liked him. He was 
confident, dashing. Greenwood carried some 
sort of an electronic device which we were 
told he built himself, based on plans he 
found on the internet"
Harold Greenwood

Trespasser Storyboard image of Greenwood

Hidden inside the Dam on the Trespasser version of Isla Sorna a skeleton can be found. The deceased starved while hiding from Velociraptors, after he'd managed to crawl into the dam losing blood.

While it's still speculation to assume the skeleton and Harold Greenwood are one-in-the-same, it's generally accepted. Harold probably was a spy of Biosyn who came to the island with the crashed Biosyn helicopter that can be found elsewhere on the island; although, according to some unused sound files he was a U.S. Marine.