Allow users to subscribe (double opt-in) to a CiviCRM group without creating a Drupal user. Upon entering an email address, the user is added to the group, but the do_not_email flag is set. The subscribe form sends out an email to confirm the subscription. And only after the special link in the email is clicked, is the do_not_email flag cleared.

See also CiviCRM ActiveUser

This module is partially supported by CivicActions.