While it does not match your desired criterion of being .NET-based, I highly recommend running WordPress in "multisite" mode:
- Each organization in the community would get its own site (in the "network" of sites), with its own admin for that site. (The individual sites can live in either subdomains or subdirectories relative to the root-domain.)
- The "super-admin" for the entire "network" has admin privileges for all of the organizational sites of the community.
- In addition to being very powerful, flexible, and extensible, WordPress is considered to be relatively user-friendly, out-of-the-box, as well. But there are quite a few plugins (such as 'Adminize') that allow the super-admin to simplify the admin dashboard even further, in a centralized manner, for all of the sites.
- The WordPress.org core, together with all of the numerous plugins featured in the official Plugin Directory, are all open-source, under the GPLv2 license.
Also, although you did not mention this as a requirement, WordPress gives you the option, via the BuddyPress plugin, to create a Facebook-like social-network, where each of the organization members can have their own personal profile and socially interact with other members.
Likewise, there are numerous calendar-related plugins that organizations could use to publicize upcoming events.