gistfile1.php

/**
 * Implements hook_views_data_alter().
 */
function feature_user_views_data_alter(&$data) {

  // Actions
  $data['users']['profession'] = array(
    'title' => t('Profession'),
    'help' => t('Profession of a user'),
    'field' => array(
      'handler' => 'views_handler_field_user_profession',
    ),
    'filter' => array(
      'handler' => 'views_handler_filter_string',
    ),
    // This will show a term relationship on a user view
    'relationship' => array(
      'handler' => 'views_handler_relationship',
      'base' => 'taxonomy_term_data',
      'base field' => 'tid',
    ),
  );

// If you need to join automatically use that
  $data['taxonomy_term_data']['table']['join']['users'] = array(
    'field' => 'tid',
    'left_field' => 'profession',
  );

}