Well first off, I would go by the method that phpBB has to create a user. Try this function to create one.
http://wiki.phpbb.com/Add_users
Now, to load phpBB's functions...
Since your saying you file is located in:
/home/
And you forums directory is:
/home/forum/
phpBB requires that you define a few things before you load it. You need this:
<?php
define('MY_ROOT', dirname(__FILE__)); // This the the AutoReg.php file location.
define('IN_PHPBB', true); // Checked by phpBB
$phpEx = substr(strrchr(__FILE__, '.'), 1); // Needed by phpBB
$phpbb_root_path = MY_ROOOT . '/forum/'; // The forums root.
include($phpbb_root_path . 'common.' . $phpEx); // The file loader.
include($phpbb_root_path . 'includes/functions_user.php');
include($phpbb_root_path . 'includes/ucp/ucp_register.php');
// Now do what you please....
From there, you can either user that user_add() function or whatever you would like.