<?php

// In my extension it was good enough to hardcode in a constant. If desirable, one or more special group
// IDs could be stored in a CiviCRM setting.
define('EXAMPLEEXTENSION_GROUP_ID', 44);

/**
 * Implements hook_civicrm_post().
 */
function exampleextension_civicrm_post($op, $objectName, $groupId, &$contactIds) {
  if ($op === 'create' && $objectName === 'GroupContact' && $groupId == EXAMPLEEXTENSION_GROUP_ID) {
    foreach ($contactIds as $cid) {
      try {
        $email = civicrm_api3('Contact', 'getvalue', array(
          'id' => $cid,
          'return' => 'email',
        ));
      } catch (Exception $e) {
        continue;
      }

      // it's theoretically possible that a Drupal user exists with the given email
      // address but that it doesn't have a UFJoin, so we'll check for the existence
      // of the actual account
      if (!user_load_by_mail($email)) {
        $drupalUser = _ee_createAccount($email);
        if ($drupalUser) {
          $operation = 'register_no_approval_required';
          _user_mail_notify($operation, $drupalUser);
        }
      }
    }
  }
}

/**
 * Creates a Drupal user.
 *
 * Assumes that the nonexistence of the Drupal user has already been validated.
 *
 * @param string $email
 * @return mixed
 *   Drupal user object if a new user was created,
 *   FALSE if creation failed
 */
function _ee_createAccount($email) {
  $result = FALSE;

  $params = array(
    'init' => $email,
    'name' => $email,
    'mail' => $email,
    'pass' => md5(uniqid(rand(), true)),
    'status' => 1,
  );
  $result = user_save(NULL, $params);

  return $result;
}