At the start, I asked students to form groups where everyone is a stranger. I change the groups every 3-4 weeks. It helps students to know that arrangements are temporary. The key factor at present for me is to form a group which can regularly meet outside of class. Groups formed by students are usually successful.
Some students are reluctant to participate at first, but usually join.
I have left groups in place for 3 or 4 weeks, not long enough to build group identity. I have tried letting the students name several people they want in their group and I promise to try to get at least one of these people in each person s group. In courses where I regularly give group quizzes, the students may choose any partner except one they ve worked with before. So mostly I try to get students to keep moving. One of my goals for group experiences is for students to learn from their fellows.