At this point of the year, you are probably doing more and more work in cooperative groups. If you haven't identified specific roles and responsibilities for the students in each group, they will determine them for you. Some will take on the roles that you would have created and intended while others will find more passive or aggressive roles that you would probably prefer to avoid.
One way of making group work more productive is by assigning roles that will change with time. In my classroom, I tended to change the table groups every month. Based on the location of the desks within each table, the students took on a specific role for one week. For example, the person at each table that was sitting in the Northwest corner (I always posted North, South, East, and West on my classroom walls) of their table would be the Leader/Speaker for his/her group for an entire week. The following week, all of the roles would shift in a clockwise manner. The Leader/Speaker would now become the Facilitator. Clear as mud?
If you are thinking about introducing this idea of roles and responsibilities for group work to your students, I would highly recommend reading this book:
Todos tienen su tarea.
Todos son felices.
The illustrations are beautiful and the language is brilliant. You could do an in-depth study of vivid verbs with this book! It's a great one for any elementary grade! Let me know if you and your class decide to try it out!