The first thing that comes to mind for out of game communication is the steam group chat. However with there being 3 different groups with people in all of them, I would suggest Slack, it's free, easy to setup and easy to use. It has decent plugin support and the ability to ping certain people or channels; you can also setup you're personal client to highlight or ping when certain words are spoken (i.e. CSGO, TF2, Gmod).