"everybody sees that someone joined" i can see that xxx has joined. Is the message coming from server or client? If it is from the server, then the attacker could block that message right?
When someone joins/leaves the group every participant generates and distribute a new key. So as a rogue participant, the keys you’ll receive won’t allow you to decrypt past messages.
PS: this article was posted before the speaker was even done with his talk. This is funny?
However, it's clear that the severity will depend on the particular group. A group of three friends will certainly notice when someone is added to the membership. A larger group of people who don't know each other personally but trust the administrator to administer membership will have to rely on the admin both noticing and taking action, and he may be hampered in doing so by the attacker.