From LQWiki

The group concept enables the administrator to assign permissions and restrictions based on groups of users.

Linux implementation of groups uses a textfile (/etc/group) to define groups and map group names to GIDs.

The group file consist of four fields:

• Group name
• Password - Encrypted password, if empty no password required
• GID
• User list - Members of the group, separated by commas

See also

To change the group ownership of a file or directory, use the chgrp command.

To add a new group, use the groupadd command.

To add a user to a group, use usermod, or in useradd at user creation time.