Often when students work in groups, one or two of them end up doing all of the work by themselves. The other people may just chat and fool around knowing that the "hard-workers" will get the work done. In order to avoid this, have everyone in the group assigned to a specific job. You can choose jobs based on the number of students you want in each group, and then they can elect people for each position. You should have the job titles and responsibilities written out to save you time from continuous explanation. The students should write their names next to their particular function so that you will know if everyone did their fair share.