I recently downloaded Dynasty Warriors 8 Empires (DW8E) from XBox Live. My roommate and I both created characters under our Live gamer tags. I log in and start Empire Mode and select my created character. Start the game and he can't seem to join. Have tried it on the Pre-Battle screen to no avail. Tried doing it on Free Mode with multiple officers, still nothing. Ideas?
Here's how 2P works on the DW8E, at least on PC after current update (PC version 184.108.40.206):
In Free Mode Invasions, Defensive Battles, and Raid Scenarios, 2P can join in by pressing start (or F11 on PC, for whatever reason) at the setup screen where you set the difficulty, officers / marching officer, and so forth. Look for the "2P Press Start" prompt at the top of the screen. Event Battles and Quests cannot be done in 2P mode.
In Empire Mode, in any offensive or defensive battle in which you can select officers, 2P can press start to join in for that battle (again, look for the 2P Press Start prompt at the top of the screen). 2P will control the SECOND officer in the list (after the one controlled by 1P). Quests and Event Battles, since they do not permit you to select the officer you are controlling, do not allow for 2P.
Whether all of the above applies to the Xbox One version depends on whether the equivalent updates were pushed out to the Xbox One version. I'm fairly certain the PC version has feature parity with the PS4 version except for online multiplayer.