How *.drupal.org single sign-on works
You might have a look at Bakery Single Sign-On System. 

It mentions "This module started at the 2009 Drupal.org redesign sprint in San Francisco as a way to manage single sign-ons across *.drupal.org infrastructure."

For more details about how it work, you can see this article that has great explanation: Single Sign-on across Sub-Domains in Drupal with No Extra Modules.