Do this ---
Open the Scenario -> Player Properties
Now you see the players names , colors , races and controller. Open up the controller in player you want to make "computer" and make it neutral or computer. Now if you want to be ally with that "computer" you should open the "Forces" tab. Now you should see Force 1 and Force 2 (if you added computers) name force one in example good , and force two evil (this is example) now put the amount of players you want and a "computer" in each force , and check Allied , Allied Victory and Share Vision for both forces. Now you have a computer in force "good" , who is friend to other players in "good" and the same with force "evil". If you have questions , or your problem isn't solved , PM me , I will answer , or post screenshots how to do that.