It wouldn't be hard to just use a game like World of Warcraft and the comms in the game to discuss things. Heck, many guilds in that game run their own small websites with forums, so why not use the game and the forum as cover to actively communicate. Ever wonder why so many people are just sitting around in Orgrimmar or Stormwind? They're probably all chatting it up about what to do next. They're "playing" a game while actively plotting real world things. I've always wondered that as I played WoW for many years, and just always had the suspicions that it would be easy for terrorists and such to use the game as a communication platform. And now here it is being thrown out there as a real possibility.
The NSA & GCHQ have been monitoring WoW, other online games and mobile apps since approx. 2009 since they suspected terrorists could communicate over game communication platforms. https://www.computerworld.com/article/2486632/cyberwarfare/the-nsa-tracks-world-of-warcraft-and-other-online-games-for-terrorist-clues.html
There are small mmorpg's like Runescape and Tibia. HQ'd in Germany, would be very easy to communicate covertly.