Your behavior will say more than any words. Assign roles to students. Roles can encourage the practice and development of civic values. The role of encouraging the participation of each group member carries the message that all group members' contributions are important. It takes a lot of skill for students to manage a group of 4 or more.
Smaller groups are more effective and take less time.
Not preparing students to work in cooperative groups. Explain to students why you are using cooperative learning, do a short cooperative learning activity, then have them explain how it can help them. Initially, do short get-acquainted and review activities. Not teaching students appropriate interaction skills. Ask students to contribute to a class list of appropriate group behaviors.