(this will throw an Error in a future version of PHP)

[.m.]

look around line 36 of the plugin file viewgroups.php

if(THIS_SCRIPT == 'index.php')

change it like below & check if it fixes referred issue

if(defined("THIS_SCRIPT") && THIS_SCRIPT == 'index.php')

Note: php files should be saved with utf-8 encoding without Byte Order Mark.
in general, we use editor like Notepad++ or file editor at web host panel to edit the files.