Below is a brief explanation on how it is known Ben Franklin was a Rosicrucian along with a link to the Rosicrucian's offical website and their claim to his membership.
http://www.rosicrucian.org/about/mastery/mastery08history.html
http://www.geocities.com/Athens/Acropolis/2216/Ephrata_Presentation/56_Ephrata.htm
Benjamin Franklin was born in Boston
[1706-1790] We all have wondered how it is known that Benjamin Franklin [1706-1790] was a Rosicrucian. Well, it is partly by reading what he wrote—through his writing, we can recognize his Rosicrucianism. However, Manly P. Hall wrote that Franklin's signature appears on the ledger—above that of Marquis de Lafayette, he pointed out—at the Lodge of Perfection in France. That Lodge is considered to have been "the greatest of all French lodges" as Hall wrote it. In The Secret Destiny of America, he said of Franklin in America:
"Franklin spoke for the Order of the Quest, and most of the men who worked with him in the early days of the American Revolution were also members. The plan was working out, the New Atlantis was coming into being, in accordance with the program laid down by Francis Bacon a hundred and fifty years earlier.
"The rise of American democracy was necessary to a world program. At the appointed hour, the freedom of man was publicly declared."
Imperator Gary L. Stewart, in 1984, stated that it is known that both Rosicrucians and Martinists attended that French Lodge of Perfection. The Order of the Quest is another of different spiritual Orders tied with the Rosicrucian movement of initiatic individuals, that functioned generally in connection with the western esoteric tradition. It was sometimes referred to as the western mystery tradition. Brethren of the Rosicrucians were usually members of more than one secret society. Chapters of Perfection included leaders from religions and scientific organizations, as well as esoteric and humanitarian orders.