How to manage users in multisite

I'm working in a users community project using multisites.
I want that a registered user on the main site could have it's own site and manage it.
Also I want that the main site could have access to news/content from the children sites.
Users could be member of several communities (multisite), that´s why I share users table.
Each site has:
* it's own database and only share users and sessions tables (which are in a different database)
* roles like founder, administrator and community member
Which is the way that the administrator of a community could only manage users that are registered in his community and not all users (which came from sharing users table)?
Should I share any other table, give permissions to the role or there is a module that do that functionality?

Thanks in advanced and I'll wait for your suggestions.