Erik, you use the Invaders in the Arma Mission Editor, when you start the game and your at the main menu, click the button that says Editor, then pick an Island and press ok.
So then you double click on the map, and you should get a menu that lets you put units on the map, click on Blufor,( which is most likely what side it will be on,and then you click Independant. the should be something like XCOM or something, all i remmber is the aliens start with a big X, and then youclick on that and you press ok. ten you click Preview, after that you can make alot more Aliens by double clicking n the map again and doing the same thing.
If this is too long please tell me so I will know not to make posts this long in the future.