The procedure is pretty much the same as D:OS EE, if you've played that.
When starting the game, if a controller is detected, there will be a prompt before the main menu to press a button or key to choose the input method (can be changed, at the main menu, but not while a save is loaded). After pressing the controller button and starting a new game, or loading an existing save, the other player would hit the menu button and then select a profile in order to join. You would have to at least set up a profile for the host (the client may be able to select an automatically generated 'Debug_Client_Profile_1' profile; have not checked in the dev version, but that was created in Early Access when starting a second instance of the game [to test LAN co-op, or create multiple custom characters during character creation]).
If someone joins during character creation, you would each customize your own character, otherwise they would take over a character in the party.
