clipped from: www.law.umkc.edu   
The Trial of John Brown: The Secret Six

Theodore Parker
Liberal Transcendentalist Boston minister Theodore Parker was a strong supporter of Slave revolts, women's rights, temperance, and prison reform.  His radicalism fueled by enactment of the Fugitive Slave Act, Parker participated in attempted rescues of fugitive slaves and formed a vigilance committee in Boston to fight those seeking to capture fugitive slaves.  Parker wrote at the time of the Harper's Ferry attack, "One held against his will as a slave has a natural right to kill everyone who seeks to prevent his enjoyment of liberty."
Dr. Samuel Gridley Howe
Thomas Wentworth Higginson
Franklin Sanborn
Sanborn first provided money, arms, and books for Brown's activities in Kansas, then later at Harper's Ferry.
Gerrit Smith
He became so fearful that his connections to Brown would be exposed that he spent time in a lunatic asylum. 
George Luther Stearns
Stearns hosted "the John Brown party" to celebrate the Emancipation Proclamation