Both drupal.org and groups.drupal.org are using the distributed authentication feature in Drupal 5's drupal.module. This module isn't in Drupal 6, but D6 supports the concept of "identities" because it uses OpenID. Those identities are listed on user profiles and could conceivably be links to other Drupal sites, but there currently isn't an OpenID provider module in Drupal core.
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distributed authentication and OpenID
Both drupal.org and groups.drupal.org are using the distributed authentication feature in Drupal 5's drupal.module. This module isn't in Drupal 6, but D6 supports the concept of "identities" because it uses OpenID. Those identities are listed on user profiles and could conceivably be links to other Drupal sites, but there currently isn't an OpenID provider module in Drupal core.