Groups are a object type which allows users to be grouped together
    for various purposes. The main interest in this is to maintain
    security, by granting access rights to groups instead of individual
    users.

    By default, all users are in a group called EVERYONE. If you grant
    [RF] to group EVERYONE in a directory, then every user in that group
    will hold those rights in that directory.

    Some software also makes use of groups (for example, Pegasus Mail
    does not run if the user is in group NOMAIL).