If you're an Xbox player who owns Halo Infinite, or simply a Game Pass subscriber, you can (as of July 15th, 2022) participate in the Campaign Network Co-Op Flight, as relayed by Halo's official Twitter account. This will allow you to invite your online friends to your Fireteam, or join someone else's and traverse around Zeta Halo's open areas with up to 3 other players.
Players that own the game through Steam could participate as well, however, there, the beta is invite-only, and it is currently unclear if 343 Industries will continue to send out invitations during the flight's active timeline. The flight is slated to last until August 1st, 2022, 10AM PT. The beta was previously slated to begin July 11th, however, it was delayed a few days ago, with the developers simply stating the flight had issues.
Halo Infinite was officially released on December 8th, 2021 for PC and Xbox platforms. Though the game wasn't universally acclaimed to the highest degrees, most reviewed it favorably and deemed it a return to form for the series. However, history repeated itself with many complaining about the lack of a PVE multiplayer mode for the game's campaign.
Co-op and Halo went hand-in-hand during the series' biggest era, that being the original trilogy, and Halo Reach, all of which were designed by Bungie. Many Halo fans reminisce playing through Halo's vast stories with a friend or family member. Even in Halo 4's campaign, which was 343 Industries' first project within the franchise, co-op was available but was then removed in Halo 5: Guardians for unclear reasons. Furthermore, it was never added to that game, but it's good to see 343 listening to their player base's feedback now.
To actually enter the flight and try this beta out, it's not as simple as just accessing your game. You need to download the Xbox Insider Hub app, and find the 'Halo Infinite - Insider' program under where it says 'Available.' You have to join to register, which may prompt you to check back later, but once the registration is accepted, you can join Halo Infinite - Insiders group and download the flight. Once you do, it's as simple as inviting your friends to your Fireteam and loading up the campaign. Further details can be read here.
Halo Infinite's current state as an ongoing multiplayer game is not exactly optimal. After the initial hype died down, many began to criticize what they found to be a lack of content and relevant features for the multiplayer, and as time has gone on, it seems they've been slow in providing these updates, which is why the player base has dwindled. However, this is a fresh and different way to play online than the available PVP multiplayer modes, so it could bring many players back into the game.