This plugin allows you to add registered user to a new Sandbox.

motopress-demo-add-user-to-blog.php

<?php

/*
 * Plugin Name: MotoPress Demo - Add User to Sandbox
 * Plugin URI: https://motopress.com
 * Description: This plugin allows you to add registered user to a new Sandbox.
 * Author: MotoPress
 * Version: 1.0
 * Author URI: https://motopress.com
 * Network: True
*/

defined( 'ABSPATH' ) || exit;

function motopress_demo_add_user_to_blog( $source_id, $target_id ) {

	// select registered user
	$vendor_user = get_user_by( 'email', 'vendor@local.loc' );

	if ( $vendor_user ) {

		$vendor_user_id = $vendor_user->ID;

		// add user to sandbox
		add_user_to_blog(
			$target_id,
			$vendor_user_id,
			'administrator'
		);
	}
}

add_action('mp_demo_create_sandbox', 'motopress_demo_add_user_to_blog', 10, 2);