group is a name service

Name Service Switch

The Name Service Switch is a facility in Unix-like operating systems that provides a variety of sources for common configuration databases and name resolution mechanisms...

 database used to store group information on Unix-like

Unix-like

A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 operating systems.

The sources for the group database (and hence the sources for groups on a system) are configured, like other name service databases, in nsswitch.conf.

Seeing the available groups on a Unix system

The contents of the group database (and available groups) can be seen with a variety of tools:

Fetching a specific group

For a specific group called 'users':

getent group users

Python

• grp - The Group Database — a Python
  Python (programming language)
  Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
  
  module