If you wish to matchmake, make sure this is on. You should see underneath those slots a prompt that says to press Square/X to turn Matchmaking on or off. Outside of the hero roster menu, you’ll see your selected hero, your AI companions (if any), and whatever open slots you have on your strike team. If you want AI Companions, hover your cursor over a hero in the Hero Roster menu and press Square/X to set a hero as an AI Companion.Select the hero you wish to play as in the Hero Roster menu.After selecting a mission, you'll be taken to a new team - there will be two options: Start Mission and Hero Roster.To open up Matchmaking, select a mission that allows Multiplayer.