I am certain that Sayid told Charlie what he did because A) he knows Charlie helped Sawyer steal the guns, B) he knows Charlie can probably get in undetected based on that, and C) he knows Charlie has the balls to off this guy, because he iced the dude who took Claire in cold blood.
One thing that's annoying me is how everyone knows that Jack is forming this army, but no one is in it. There's just something charmingly inept about hanging out in the woods with a girl you like and calling it "army practice".
He didn't ask Locke, Sayid, Sawyer, Kate, Ecko, or Jin to be in it. So it's him and his girlfriend...and like, "Steve" or whoever else they mentioned that we don't see...
and so far they've managed to accomplish nothing... except for losing all their guns...
http://www.thetailsection.com/