Users on Linux systems are assigned to one or more groups for following reasons:
• To share files or other resource with a small number of users
• Ease of user management
• Ease of user monitoring
• User Groups can solve many problems related to large Linux (UNIX) installation.
• Group membership gives you or your user special access to files and directories or devices which are permitted to that group
A new group with the same name as the user is created by default when a new user is created in a RedHat Enterprise Linux. This new group is referred to as a private (or primary) user group.
Every user has a default group, which is usually the user’s private user group, but every user can also be a member of more than one group.
A unique integer known as a GID is associated with each group. GIDs below 500 are reserved for system groups.
Apart from the primary group, a user can be a member of another group also, which is known as supplemental groups.