The server moderator creates the community, or gives users permission to create communities on their server. I have no problem with that model.
Hosting servers basically have 4 functions:
- Registering users
- Authenticating users to other servers
- Hosting forums
- Pushing/Receiving notifications
Let's say you're running server at example.com, and I sign up. My username is bob. So, I'm bob@example.com.
Example.com has several local forums, but they lack a significant tech community. So, I go to example2.com, which has a large tech community. Using my login from example.com (and postMessages), I login to example2.com, as bob@example.com. Example2.com has example.com whitelisted as a valid authenticator. So, it will accept their users.
Let's then say that Bob gets a reply at example2.com. Example2.com pushes a notification to example.com, where Bob can read it.