I seriously don't believe either side in its entirety, I think the facts will fall somewhere in the middle.
I do hope that they investigate this completely and find out what the actual facts are. I do think they should be able to get a transcript of the discussion and if blame is to be found there should also be appropriate accountability of any party(s) that bear blame.
I would say the people who have had at least three distinct stories about it are the ones a reasonable person would believe are lying.
1) It never happened
2) It happened but nothing classified was in it
3) It happened and while we say nothing is classified in it people may be prosecuted for releasing the contents.
Security implications aside we are talking about pure, abject stupidity, incompetence, and potentially criminal behavior. One of the participants was joining the group FROM FUCKING MOSCOW. These people should be thanking god they aren't headed to prison.