I have the plugin installed, default settings, I'm an admin & member of the group. I've tried [groupname] and all.
Perhaps the problem is related to my server's DNS name. My server is chat.foo.com
So, I've tried broadcasting to [group]@broadcast.chat.foo.com
It seems to accept the message, but doesn't broadcast.
However, if I send to all@broadcast.chat.foo.com it works.