A group is a collection of users who can share files and other system resources. For example, the set of users working on the same project could be formed into a group. A group is traditionally known as a UNIX group.
Each group must have a name, a group identification (GID) number, and a list of user names that belong to the group. A GID identifies the group internally to the system. The two types of groups that a user can belong to are:
Sometimes a user's secondary group is not important. For example, ownership of files reflect the primary group, not any secondary groups. Other applications, however, might rely on a user's secondary memberships. For example, a user has to be a member of the sysadmin group (group 14) to use the Admintool software, but it doesn't matter if group 14 is his or her current primary group.
The groups command lists the groups that a user belongs to. A user can have only one primary group at a time. However, the user can temporarily change the user's primary group (with the newgrp command) to any other group in which the user is a member.
When adding a user account, you must assign a primary group for a user or accept the default: staff (group 10). The primary group should already exist (if it doesn't exist, specify the group by a GID number). User names are not added to primary groups. If they were, the list might become too long. Before you can assign users to a new secondary group, you must create the group and assign it a GID number.
Groups can be local to a system or can be managed through a name service. To simplify group administration, you should use a name service like NIS+, which enables you to centrally manage group memberships.