Source

File: wp-admin/includes/class-wp-users-list-table.php

public function __construct( $args = array() ) {
		parent::__construct(
			array(
				'singular' => 'user',
				'plural'   => 'users',
				'screen'   => isset( $args['screen'] ) ? $args['screen'] : null,
			)
		);

		$this->is_site_users = 'site-users-network' === $this->screen->id;

		if ( $this->is_site_users ) {
			$this->site_id = isset( $_REQUEST['id'] ) ? intval( $_REQUEST['id'] ) : 0;
		}
	}