The assessment criteria can provide clear guidelines to the group about effective group practices.

One option is to engage the students in writing the group contribution assessment criteria. This way, they simultaneously have a discussion about acceptable norms of behaviour in groups. For example, the following criteria were written by first year students at Flinders:

Co-operation
This person: works with the group to reach a consensus view and then abides by that consensus; works with others to finish the assignment; compromises; does their assigned tasks and does not intrude on others' tasks
Attendance
This person: is punctual; is prepared; turns up at all scheduled events
Contribution
This person: makes sure they do the work the team decided they would do; brings work back to the group by the deadline the group has set; does their fair share of the roles (e.g. chairing, editing, etc.)
Communication
This person: listens to others' ideas; is contactable (responds to emails / answers their phone); is open to new ideas; includes others in the discussion
Works efficiently
This person: has good time management skills; meets deadlines; stays focused in meetings
Responsibility
This person: if they're given something to do, then they do it; makes sure they provide good references for the material they have researched

For more on designing assessment criteria for groupwork, see http://www.iml.uts.edu.au/assessment/group/criteria/criteria.html