Group is also a
sociological term. It is quite
subjective and its exact
definition changes with any change in the
situation, but I shall try to illustrate its definition and
usage to the best of my ability. A group is usually composed of two or more
people, although in certain special cases it can be comprised of one person only, as we'll see shortly.
The scope of any given group is determined first and foremost by the members of said group and their willingness or lack thereof to include specific others within their group. An easy illustration of this phenomenon can be seen in a stage performance. Any time a person is onstage, the audience sees him or her as being divorced from them in the sense that he becomes part of a group that is mutually exclusive with the audience: the performers. If the performer leaves the stage and views the remainder of the show from the audience, acting like an audience member, he will most likely retain enough characteristics of a performer (i.e. costume, makeup, a memory in the audience's mind of his presence onstage) to preclude his acceptance as a full member of the audience. The audience finds it necessary to create its identity both through self-definition ("Audience members sit in chairs quietly.") as well as other-based definition ("Audience members are not performers."). He will probably be on the recieving end of behaviors that the audience normally reserves for persons who are not members of itself, such as stares, congratulations and awed handshakes. In other words, the audience maintains its membership and border integrity by failing to allow persons that it does not recognize as members into the group.
Not every group's boundaries are this rigid, however. On a bus, membership in the group of passengers is as simple as paying a fare. Once the basic requirements for membership in any group are met, whether those requirements have to do with a person's race, social standing, or ability to pay a dollar and a half to get on the bus, the person is almost always accepted as a full member of the group and becomes subject to its rights and obligations, which it is necessary to meet to remain a member of the group.
Another very important fact concerning groups is that every person has numerous group memberships at any given time. All of these memberships are, however, not equally relevant at any given time. For a man involved in the 1991 Los Angeles riots following the Rodney King beating, racial membership will be almost certainly the most important and relevant membership to him, while his status as a Rotary club member will probably be forgotten rather quickly. This does not mean that he is no longer a member of the Rotary Club group, but it does mean that racial category overshadows the Rotarians at that time in both the mind of the subject as well as the mind of the sociologist studying him. In this sense, the man can be taken to be a representative of said group, and especially if he is the only one of its members present, he can sometimes be said to constitute something of a group in himself when seen from an outside point of view. Reginald Denny, for instance, was the only white man present at his beating and therefore could be said to be a member of the "white group," although he is the only member present at the time. This is an especially vivid example of group definition by exclusion, as the group of black men in the news footage that were hitting him with bricks derived their group membership partially from contrast with the nonmember, to whom behavior was shown that completely precluded his possible membership in their group.
I believe that this covers most of the basic characteristics of groups, group formation, and group membership. I intend to talk about intergroup conflict, intragroup conflict, as well as a few other possibilities regarding special cases of group membership and definition in other nodes that I have not yet made. If there's anything else that you feel should appear in this writeup (as the possibility of my having missed an important something is quite distinct) please /msg me and I'll see what I can do to correct the omission.