Drupal Schools Network

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.

So having a dev.mysite.com and mysite.com as two multisites is always a good practice, putting the modules under development in /sites/dev.mysite.com/modules (so they don't bring down your live site).