require_once  MYBB_ROOT."inc/datahandlers/user.php";
$userhandler = new UserDataHandler("insert");
$user = array("username" => $username,
    "password" => $password,
    "password2" => $password,
    "email" => $email,
    "email2" => $email,
    // ... (extra fields)
);
$userhandler->set_data($user);
if($userhandler->validate_user()) {
    $newuser = $userhandler->insert_user();
    // do whatever you want with the $newuser array. It contains the new user's data.
}

$hashedpsw = md5(md5($salt).md5($plainpassword));
/* $salt = random 8-chars long string
 * $plainpassword = the password in plain text
 * $hashedpsw = the hashed password
 */