Discord is a messaging app built for gamers and it is available for installation on mobile (android/iOS) and desktop (windows/macOS), or can be accessed through any web browser at http://discordapp.com. It can also be installed on the Xbox dashboard.
Our company uses a bot to monitor member's activity and stats. Members also access the bot to monitor their own activity and scoring stats using Discord.
It is an easy way to put together or join a fireteam as we share the the Discord server with several alliance companies. (There are voice channels you can use instead of party chat on the xbox.)
It's a great way to connect with other Halo players.