In Linux, each user is assigned at least one group. All the information related to groups such as group name, group members, and group ID are stored in /etc/group file. By default, when a new user is created, a group with the same name as that of username is also created at the same time and the newly created user become a member of that group. You can modify this behavior as well.
There are four fields per line entry and are separated by a colon(:). Each row in this file represents entry of one group. Below image is a sample entry from group file-
Note: More than one user can become members of a single group. A user can be a member of several groups.
The above line shows group information for group sysadmin and below it is explained in detail-