Application handling multiple organisations each with multiple members


I am looking at an application that will provide a service to many organisations. Each organisation may have many members able to login.

The data for each organisation is separate and should not be seen by other organisations. There needs to be two roles for members. An admin role and a member role.

I will have to write some code for the custom application.

Is there an existing application framework that already caters for this scenario?

I have used Joomla! in the past, but that doesn’t seem to support it.

I wonder about WordpressMU.

I usually use mysql/php, but will consider a .NET solution if one exists.

Thanks for your help,