Ready up players to queue matchmaking
To queue a player or a party up for matchmaking, you need to have each player indicate their readiness.
Once all players in a party are ready, or if you're not in a party and the only local player is ready, matchmaking will begin.
Queuing solo, or parties with no split-screen
If you're queuing a solo player locally, or if you're in a party but don't support split-screen in your game, you can call "Set Ready State" with a default user ID in the Matchmaker. It'll automatically select the user ID of the first local player controller.
Queuing parties with split-screen
If you have multiple local players (such as on console), you'll need to tell the matchmaker which local player is readying up for matchmaking by passing their user ID.
Unreadying a player
To unready a previously readied player, just call Set Ready State again with the same arguments, but with "Is Ready" set to false.