BuddyPress.org

Function doc says:
* @param array $result Results of user validation including errors, if any.
Original function fires

$result = array(
	'user_name'  => $user_name,
	'user_email' => $user_email,
	'errors'     => $errors,
);

Try

function validate_user_signup( $result ) {
  // the modified code and at least something like
  return $result;
}
add_filter( 'bp_core_validate_user_signup', 'validate_user_signup' );