This is cheap and easy. I made shirts for my husband, son, sister, niece, and myself. We couldn't find a solid shirt in my niece's size, so I got a hoodie, and cut the M in half and did one on each side of the zipper. All together we were a "pack of M&Ms."
I did Ms on the front and Ws on the back, cause that's what they look like, and had to listen to people call us "M&Ws" all night. So you may want to do Ms on front and back, but then people will tell you how wrong it is, so if I did it again, I would only do it on the front.
You can put a little more effort into it. My son was the red M, and I sprayed his hair with temporary dye, and made him wear his read Converses. I was the blue M and put the effort in to wear "blue" jeans. I also had blue eyeshadow, and blue lipstick, that's why I look a bit frost bitten in some of the picture.