Groups : to show the users that belong to a group …

The easiest way find all of the members of a group is to install the package members.

sudo apt-get install members

EXAMPLE:

members --all groupname

the above will list all of the members of groupname.

Usage: members  [ -apst --all --primary --secondary --two-lines ]

To show a list of all groupson your system, just type

groups

To show all groups a user belongs to, just type

groups username