Agreed that this is a sorely missed feature that you'd normally find in other games that include guilds. It's important to have a channel to discuss matters to which you might only want certain members privy.
Right now, the only alternative is to use another app such as Line or Kik, which requires another separate download and takes up space on your phone or tablet. It would be nice if chat offered some sort of group conversation. And about a million more enhancements, like new messages showing up in the main window at the top.