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. If this is the beginning of the course, this may be a reasonable approach since it is non-threatening. Alternatively, the instructor can simply rotate roles within the group after the first activity to assure that all students experience a multiude of roles. This is especially useful if there are a series of week-long tasks for the group.