No problem, thanks for the clarification of your question..
1) Potentially you could look at a plugin such as
Network Privacy which would allow such a thing, perhaps you could look at the plugin details and see if this is what you had in mind?
2) If you restricted registration to only those accounts you as the admin set up, then you could certainly restrict who can post and set permissions as you create them. Or, if you left the registration open with a limited permission level for access by default, then you'd need to manually give specific users the ability to post by changing their permissions. Ultimately the current setup would require some kind of manual intervention for users. Thanks.