CiviCRM Active User
civicrm_activeuser is a simple module that adds a user to a group upon Drupal login.
This was developed primarily to support a double-optin check when using CiviMail. Not only must the user signup for a CiviCRM group, they must use their Drupal login by signing in at least once.
See also CiviCRM Subscribe
This module is partially supported by CivicActions.