Manual:EmptyUserGroup.php

Details
emptyUserGroup.php file is a maint-scripts>Special:MyLanguage/Manual:Maintenance scripts|maintenance script that removes all users from a given group.

The only parameter, in addition to, is name of the group to be removed.

See below section for example:

Usage
If name of the group contains whitespace, it should be enclosed in double quotes ("), as shown in example.