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Discord is a chat and voice service similar to Slack or Hangouts but aimed more at the gamer. There's also an app for your computer and mobile phones. So you can chat with others while away from game. The text chat is separate from in game chat though voice chat could be used in game overlay.
If you are a whitelisted member, log into the forums for an invite link.
Below is a list of current online chatters. Mobile users will receive notification of posts to the chat and also you will receive notice of new forum posts.