Suggestions on CMS with multi-site support

I'm looking for a CMS or similar system with the following requirements for a project I'm helping out on. The basic idea of the project is to allow users in completely separate groups to have their own communities (sites) where they can do things such as book facilities, create and vote on polls, disc开发者_如何学编程uss on a forum and so on.

All CMS's probably have these features, except for the multiple separate groups part. As there will likely be too many groups for our admin to handle, all group admins should be able to manage their own sites (polls, facilities, users etc.). Different sites will share the layout and design. The available "modules" are installed and made available by admins.

Does such a CMS exist, or should I start inventing my own using a framework like Django or MVC.NET? Coding extra extensions to an existing CMS won't be a problem if one which satisfies our basic needs can be found. A good extension framework is a huge plus of course.

I've used Joomla in a different project before, but it's not what this project needs.

If you have any suggestions, please point me in the right direction, thanks!

I would recommend you to check Drupal and Organic Groups module.