Custom plugin modified results in WSOD PluginNotFoundException
Plugin definition its a tandem between the plugin manager wich defines how your plugin type should be discovered and instantiated; and the plugin definition method itself that could be either using YAML files or annotations. Depending of which you used to define you new plugin type is how you must proceed in order Drupal find your plugin implementation. Said that if your plugin is used in a configuration entity you can find it querying your configuration table as follows:

SELECT * FROM config WHERE name LIKE '%group_user%' OR data LIKE '%group_user%';

In the other hand if your plugin is invoked directly in code by calling the plugin manager method createInstance you can then find where it is being used with a simple search in your custom code using your favorite editor or IDE. I emphasize on custom code as is a custom plugin type cannot be (yet) in contrib code.