Drupal Login Synchronization
The Drupal Login Synchronization plugin shares user accounts between a CWIS installation and a Drupal installation, such that logging into either site automatically logs a user into both. In addition to enabling the CWIS plugin, a companion Drupal plugin must be installed and enabled on the Drupal site for synchronization to function. Accounts which exist on the CWIS site but are absent on the Drupal site will be created as needed. However, preexisting accounts on the Drupal site (from before the plugin was enabled) are not currently synchronized. See plugins/DrupalSync/README in your CWIS site for detailed instructions.
Both sites must be able to access the database server used by the CWIS site. The simplest way to ensure this is to run them on the same server.
If your sites run on different servers, you may wish to use the OpenId Server plugin instead. However, OpenId Server does not automatically log users in to all connected sites. It only allows the same user account to be used across several sites.