I know that some people say that Kerry wasn't at the meeting and others say he was but voted against the assasination plot etc.
To me the fact that he was part of the group at all should be enough.
He was more than just a part of the group; he was their co-founder and spokesperson.
Once the FBI files were made public which placed Kerry there, the Kerry camp admitted he was there.
As to whether he voted against the idea, the mere act of participating in the vote implies that he would abide by the outcome of the vote.