how do i create a site that allows users to create "organizations?
The module to do this with in Drupal would be Organic Groups https://drupal.org/project/og. It's not a trivial module though. Here's the official documentation: https://drupal.org/node/1114858.

There are some online courses available from for example Lullabot: http://drupalize.me/series/organic-groups-drupal-7
and Brian Lewis: http://modulesunraveled.com/organic-groups-7x-2x.