Ok, how can I put multiple copies of myself in one video like in this video?

I want to make a Harlem Shake. If you are wondering what a Harlem shake is, watch this video click me I want to know how to do what he did, if you look you can see he has about 5 of him. I did a thing on Facebook where I said if I get 20 likes I will do a Harlem shake video. Well I got 20 likes and I don’t want to let anyone down in embarrassing myself. So please help me out!