Co-Op Mode: If you want to add bots to a local game and play in 'Co-op' style with a friend against them, first make sure you're connected to the Internet, and using your Online account start a singleplayer game. Write down the IP number shown during the loading screen. Then, on your friend's computer once again make sure they are connected to the Internet and using an Online account, then go to the Multiplayer screen and under the Internet server browser click the 'Join IP' button and enter the IP address you wrote down earlier. You should then be able to play with each other against bots on the 16 player singleplayer maps. Thanks to AverageNewbie for this tip.