/etc/group file contains all groups available on the Linux systems. You can read all groups available with cat /etc/group command. To find out into which linux group you belong to you can use a following command:

groups

another way to access your personal group information is with id command:

id

id command will return group names you belong to as well as group numbers. to add a user to a group on your linux systems type ( only as a root ):

# usermod -G group-name user-name

Previous command will add user-name user to a group group-name. If the group does not exists you will get a following error:

usermod: unknown group group-name

In this case you can create a new group with ( only as a root ):

# groupadd group-name

There is also another way to add user to a group manually by editing /etc/group file, but this is not recommended.