Corporate user subaccounts?

I want to sell network sites to corporations, so the corporations can manage the users in their subdomain site. Only the users of a subdomain will have access to the private subdomain. But the existing network setup says that all WP users are subscribers to all sites in a network.

Maybe I’m not using the right terminology. Is there some sort of subaccount plugin or configuration option, so there can be master accounts who manage their subaccounts? The subaccounts should be in separate name spaces so the same name can be used by different people in different subdomains.