The Match Lobby
It is available to all the participants of the match, and all admins of the tournament with the "Report Results" permission.

A Participant point of view of the Match Lobby.
You can access it from a match page in the Organizer Dashboard:

The Organizer Match Lobby, accessible with a button close to "Videos" and "Streams"
The goal of this new feature is to ease up the communication between participants in a tournamentand for admins to give information or settle other matters like score disputes and such. Click on any message to display the date and time when it was sent.