What was the connection between the weather underground and president obama?

Barack Obama has said repeatedly that his association with '60s radical Bill Ayers, a "radical, leftist, small-'c' communist," as he recently described himself, and one of the founders of the Weather Underground, an anti-war group during the Vietnam period, was entirely in passing. Ayers was just "a guy who lives in my neighborhood," said Obama, and "not somebody who I exchange ideas with on a regular basis."

I found this on the hill

(you can google for more info)