There are many variations in group learning possible, from semester-long groups to "buzz" groups that are formed for a single day, then dissolved and reformed differently in future classes.
Some instructors prefer to assign specific roles to students in groups, such as Note-Taker, Discussion Leader, Reporter (for report back to larger class), Dissenter, Observer, or Summarizer.
One particularly effective use of groups is to integrate a team approach to larger chunks of the curriculum. Student teams which take group tests will study together and gain the perspective of all team mates (as well as think through the material as they defend their various positions on the "right" answer). One suggested method to involve team testing is to provide the test to students TWICE. The first time, they take it individually; this enforces accountability. A group test of the same questions later allows for discussion