It is not unusual for a group of 3-4 students working together to develop “leaders”, “followers” and “hangers-on”.  The first group do most of the thinking, the first and second contribute to the execution and the last group make use of the activities of the former.  In some groups the active participants will be unwilling to be used in this way and will confront the non-involved students.  In other groups resentment may simmer.  It is advisable to discuss group dynamics with your students before they become involved in any group activity.  Talk over the need to allocate tasks to each member, to rotate tasks to allow each student to develop the necessary skills and how, in the workplace, people working in a group need to manage themselves to the most productive end.  Decide how any final report is to be marked.  Will there be individual marks or a group result allocated equally to each student?  Will you mark each student's contribution or can the students award marks to their peers?  Whatever is decided, it needs to be discussed with the students before cooperative tasks begin.