How are CiviCRM and Organic groups related?
CiviCRM and organic groups can co-exist as independent drupal modules. There are also a few integration modules between the two developed by CiviCRM, for example, og_civicrm.
Rob Thorne has been working on some of these issues and trying to figure out a deeper/better integration between the two modules. He has been sending periodic emails to the group requesting feedback at al (with minimal response, so please do chime in with comments etc).
Here's a link to his recent email Improving Organic Group Integration With CiviCRM .
If this is important to you and/or your organization, please consider funding Rob to do more work on it (you can contact him directly), else it will remain the "holy grail" for the forseeable future.
As of CiviCRM 1.9, a module is included in the CiviCRM distributions which synchronizes members of Drupal Organic Group(s) with corresponding CiviCRM Group(s). The module name is CiviCRM OG Sync (civicrm/drupal/civicrm_og_synch.module in your CiviCRM installation). See Organic Groups CiviCRM Synchronization for more information.