Long before the groups are formed, we, as course designers, have to decide just what role the groups are to play in the course. In some of my courses, the groups have been the main learning context of the course. It is in their groups that students practice articulating their application of course concepts, analysing cases put before them, and figuring out answers to assignments for credit. Even some of the tests of basic comprehension of concepts are done by each group, as a whole. In other courses, groups are used only occasionally for class discussion, or for assignments, or for a class presentation.
Some small groups are disasters.
Like any other tool we use, small groups need periodic check-ups and tuning. It is always difficult to carve time out from the real subject matter of the course for such purely process matters, but even construction firms have to budget enough downtime to keep their equipment in good repair. If working well in small groups is specified up front as a significant goal of the course, spending time on non-content matters is easier. It is seen as legitimate in the eyes of both students and instructor. Periodic check-ups can take many forms, of course.