I assume you're talking about the Fake veteran's group that Kerry helped start (Fake because apparently several of the folks testifying in front of Congress claiming to be vets weren't)
If I've got my facts straight - Kerry claimed that he quit the group when the discussions went the way your mentioning, i.e. talking about assassinating US senatores.
He has been placed at the meeting. My info is that it was at that meeting where he resigned his leadership position. I presume that he voted no, but he the duty to report the conspiracy to the FBI.