Group Work Roles
When setting up collaborative learning activities, the instructor must decide whether to assign group roles or to let the group members decide. Some criteria to consider may include the educational maturity of the students, their familiarity with group work, and the available time for the activity.
Some groups function best if they decide the roles themselves. This is often true with more mature students or students familiar with group work. However, students may opt for the role they are most comfortable with and avoid the opportunity to develop other skills.
source: http://archive.wceruw.org/cl1/cl/doingcl/grproles.htm