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I've used a ton of different methods to get students into groups that are different than just numbering off or calling out names that are time savers as well.

One such example is to prepare index cards ahead of time with different numbers, colors, shapes, words, etc. on them. Hand one to each student as they enter the classroom at the beginning of class. If you'll want your students to get into more than one group during a specific class period, you can even use two (or more) different types of classifiers on each card. Then, when you want the students to get into groups, just tell them that you want them to find the others with the same color or same word, etc. (You can still use a reward for the first group to get together, if you want.)

In addition to that, you can find some great speaking games at the website below that can be easily adapted to your classroom situation.