There are currently four game modes:
Players are able to select a mission which will then put them (and any others in the lobby) into the matchmaking system. Once the matchmaking system has found an appropriate session the player(s) will do one of three things: host the mission lobby, join a mission lobby, or join a session in progress. If the player is put into a lobby, the mode selector will change to 'PLAY NOW' and begin to flash; this lets the player start the game. If the session is not full (i.e. does not have four players) then other players may be placed into the mission cell by matchmaking (assuming that the mission objective has not been completed). When playing a mission node for the first time, the matchmaking systems will not put the player in a session in progress. The number of squads already in the mission is displayed next to the mission's name and is only displayed while in Public mode.
Players are able to start a mission at any time and will not be put into the matchmaking system. In this mode only the player's contacts and clan members will be able to join (either via player invite or joining the player's session).
Players are able to start the mission at any time and will not be put into the matchmaking system. In this mode the player must invite others to join as they cannot join on their own. Note that no matter which matchmaking mode is selected, all Trials and Derelict missions are Invite Only.
Players are able to start the mission at any time. In this mode, no other players may join the session. If the player enters the options screen, the game will be paused, unlike the previous game modes. In earlier builds, certain enemies would not spawn at all.