Basically all the names are of real people.

The Culper Ring did exist, though it was organized and began operations in 1778, years after depicted in the show. By that point the British Army had marched back from Philadelphia to New York with Washington watching them from New Jersey and up the Hudson, and the main forces would esentially remain in that stauts quo for the rest of the war. Thus a source of information from inside the city was crucial.

Tallmadge did indeed become the Chief of Intelligence for Washington acting essentially as his S-2, and was actually an officer in the 2nd Continental Light Dragoons as depicted.

Woodhull though did not marry until 1781, and was indeed roped in after being caught smuggling.

Brewster was the man who piloted ships and transferred messages.

Anna Strong was Woodhull's neighbor and involved actively, though her husband was not killed and survived the war in prison, and they had 9 children.

Townsend was the agent in the City, he wrote a column for a loyalist paper, and had interests in both a tailor and coffee shop.

Washington though was much more active in the operations then is really depicted, even sending letters direct to Woodhull spelling out what information to gather and methods to use.

John Graves Simcoe was a real officer. He was wounded leading men at Brandywine Creek during the Philadelphia campaign, and while trying to recruit free blacks to the British was named the commander of the Queen's Rangers and did so with competency and mild distinction through the end of the war.

After he became the first Royal Governor of Upper Canada (Southern Ontario) in 1791. He was remarkably progressive in the post introducing consistent rule of law, founding Toronto, and abolishing slavery.

http://allthingsliberty.com/2014/04/amc-turn-everything-historians-need-to-know/

This article from the Journal of the American Revolution is a good introduction to the topic too.