/etc/group

/etc/group is a text file which defines the groups to which users belong under Linux operating system. Under Linux multiple users can be categorized into groups. Linux file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals. This method, amongst others, also enables the Superuser to delegate some administrative tasks to normal users.

File format The /etc/group file is a text file with one group record per line, each one describing a group. This record consists of four fields separated by colons. The order of these fields within the file is important.

project:x:24:smith,sara,michael

1. The first field (group_name) is the group name, i.e. a string entered in this file by the administrator or an application with sufficient privilege. The name of the group is like an identification or the record in the /etc/group file, it must be unique.
2. The second field stores information used to validate a group's password if applicable. However in the actual modern systems this field is usually set to "x", this field is supposed to receive encrypted information of the encrypted password of the group to enhance the security.
3. The third field, GID, is the group identifier, assigned by hand or automatically by the system, it’s a number that the operating system uses for internal purposes. It does not have to be unique in this file. This number can be checked in /etc/passwd file, where you conclude that each user should be assigned with a GID number.
4. The fourth field, the last one, is list of the member of the group. The names in this list are separated by commas, and they have to be in /etc/passwd.