I've been generally discounting the various text messages and emails coming out of the FBI and Meuller's investigation referencing a "secret society" as being, basically, jokes. People treading just a little bit over the political line because they know the person they are talking to feels the same way. Like in the case of Lois Lerner's messages about Republicans, it shows animus but doesn't prove wrongdoing. And it's hard to take any group of government officials referring to themselves as "Dumbledore's Army" seriously.
Turns out I was hasty.
The "secret society" started right before the gap in their message history begins. That gap in their message history starts immediately after Mueller is appointed special counsel. That seems unlikely to be a coincidence.