At first you must go to Scenario, select Player Properties, after that change boxes that say NONE to USER, COMPUTER, NEUTRAL or RESCUABLE. These will all change wheter players can enter that slot, or a computer controls a slot (good for RPGs) or even if you have a team of neutral players. The rescuable would be mainly for like the quest Save Timmy, on Arthas' first few quests. After that you may go search for an Allying tutorial, I shall be making one if there isnt allready one. You then have players/cpus/timmys/neutrals.