Search and regroup. Sometimes, after a few minutes in the group, I ll give everyone a task, set a time limit and ask them to walk around, looking for some specific things, then regroup for discussion. Encourage each person to make new acquaintances. The best way I have found to do this is to make a rule that no one can sit in the same seat, next to the same person, more than once. On field trips using vans or cars, I tell everyone to ride in a different vehicle after each stop.
People tend to be remarkably territorial.
If you leave them alone, they will always go back to their original places. But, with encouragement, they will sit elsewhere and learn a terrific amount from each other. Have a wrap-up where everyone speaks. Obviously, this gets tougher if you have groups in excess of 30 people. You will get very thought-filled replies.