Write a number on a piece of paper without letting anyone else see. Close numbers work together. When students enter the room, I offer a basket of different kinds of candy. Students selecting the same kind form a group. Like numbers work together.
Students wearing red work together, blues work together.
One day stripes work together, plaids work together, boots work together, tennies work together. Students line up and I put the shortest with the tallest, next tallest with next shortest, etc. When asking for student responses, I write the answers on the board. Because students might give false answers so that they work with a friend, I sometimes take the top student in each column to form a group rather than having all 'pizzas' work together. We find this a fun way to form groups and it helps students get to know each other.