Evidently there were twenty-one men involved in the actual attempt to seize the arsenal. It's interesting to note that the firehouse was dismantled and taken to the Chicago World's Fair in 1893 for exhibition. People were expecte to pay ten cents to tour the firehouse, a lot of money in that day.
Here's a nice treatment of Garret Smith, one of the men not present at Harpers Ferry who aided Brown --
http://www.nyhistory.com/gerritsmith/harpers.htm