The concept is simple. I created cards that each have a different color, number, and shape combination. If we are going to be working in cooperative groups during the class period, I instruct students to pick up a card on their way in. Once each student has a card, I have 3 random ways to group them - by color, by shape, or by number. They never know which one I am going to choose, so trying to cheat the system is difficult. If you are performing an activity where you need to change groups up several times, these are perfect.
If you would like to try them out in your own class, the file to download them is below. I recommend printing them on heavy paper or card stock and laminating them for continued use. Once you start using them you won't know how you ever lived without them. They are a huge time-saver and, more importantly, brain-saver!