A group is a collection of users or objects sharing a common interest or working toward a common goal. With the rapid growth of the WWW and electronic commerce, group-oriented activities have substantially increased in recent years. Examples of groups are as follows: (1) the batch of students who graduated from a high school in a given year, (2) the members of a particular travel club, (3) the students of a long-distance education course, (4) the members participating in a videoconference, (5) a set of replicated servers forming a highly available service, etc.
Groups may be classified into various types. One classification is based on whether the composition of members is fixed or variable. A ...