The devs are looking to strain the matchmaking system this time out, and since this is a beta, odds are there will be issues with the system. Halo Support has tweeted that the MCC multiplayer flight for Halo: Reach on the Xbox One is underway. The tweet points testers to the Halo support site for a list of known issues, scheduled play sessions, and to submit a ticket for any issues that players encounter.
The MCC multiplayer flight for Halo: Reach on Xbox One has begun!— Halo Support (@HaloSupport) October 2, 2019
Be sure to visit the Halo Support site for the list of known issues, scheduled play sessions, and to submit a ticket for any problems you may encounter. https://t.co/Ya2f8ZFtNS
See you on the battlefield! pic.twitter.com/gR34NohMDP
Players who were invited to the new multiplayer test have been reminded to unsubscribe from any old "Halo: MCC Insider" builds and to remove those files entirely from the console before trying to joining the new flight. Having those old files on the console can prevent insiders from launching the current flight.
While the current flight is for Xbox One players only, there are two more insider sessions for PC gamers coming. One of those PC test flights will be for multiplayer action. Those interested in joining the Halo Insider Program can sign up here. There are options for PC and console gamers. The Halo: The Master Chief Collection is on Steam for PC gamers now.