Are you sure you need a plugin for what you want to do?
Did you read about the new Roles & Capabilities at http://codex.wordpress.org/Roles_and_Capabilities#Subscriber
Of course you have to take other precautions too, such as stopping the indexing by search engines, using robots.txt.
Not all robots are well behaved :)
Reading about the "Registered Users Only" plugin, Carthik says that you may have to shut down the feeds, as they seem to get out anyway.
Another veeery old method is to use standard http Basic Authentication. You might know of the method ?
In the .htaccess file you add:
where AuthUserFile points to the htpasswd file where the encrypted user/password for each user resides.
But, I guess you don't want to know this, as it means you have to manually run the htpasswd program to add new users ;)