Group (database)

group is a name service database used to store group information on Unix-like 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.

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

The getent command can be used to fetch group information.

For a specific group called 'users':

...
Wikipedia