BuddyPress.org

Please use the code button when posting code.

You can’t over-ride that function, but if you aren’t using multisite, you can use the filter hook:
apply_filters( 'bp_core_validate_user_signup', $result );
Which only provides one argument: $result

So your function should be :

add_filter('bp_core_validate_user_signup', 'validate_user_signup', 10, 1);
function validate_user_signup($result) {
  // access to the user name: $result['user_name']
  // access to the user email: $result['user_email']
  //what is in errors: var_dump( $result['errors'] )
  return $result;
}