Server
Configuring the server for Drupal.
It depends on what you're trying to share - whether codebase, user tables, etc.
Without knowing more, I would point you to the Drupal handbook: http://drupal.org/node/346385
If you put individual sites as dot-separated-domains in the sites folder (drupalinstall/sites/my.newsite.com) and then point your virtual container (how your server decides to send HTTP requests) to the drupalinstall directory, that pretty much does it. Read more »
This page should have all the info you need: http://drupal.org/node/15365
There's also a file included with drupal called .htaccess which should have directions in it.
