Hi Kathleen:
Group study is a great way to stimulate thought, participation, and invovement. There's a natural inclusion of all in small groups, a point you made in your statement as well. Group members want to include all.
Also, as you point out, students need to stay on topic. The thing with small group study is they can disintegrate into a social interaction, off topic. So assuring progress and purpose is imporatant as a facilitator.
Regards, Barry