Joomla appears to be appears to be a wonderfully packaged web development service. Before I purchase the membership, I would like to know if there is an extension or plug in that will allow me to have individual users receive media documents and manage there own libraries without having access to the back end. I would like to reserve the right to the back end solely for the site admin.
you have chosen the most complicated portion of Joomla to decide on whether you will go ahead, or not!
Sure there is software that can help you - ACL Manager will do it all for you, but you first have to understand Joomla.
If you do not understand Joomla, ACL Manager will not help.
Look at Cloud Access tutorials on You Tube. They have a very recent tutorial on this (posted last week).
Watch it and then decide - not easy.
There are also extensive tutorials regarding this in the Pro tutorials, if I remember correctly. Really good stuff, which you should surely do before doing something like ACL Manager.
Lesson 5 of the Beyond the Basics course deals with managing Users and how to define what they can and can't see in your Joomla website. You'll learn about Security, Permissions and Groups. You might want to view this lesson to see if Joomla's built-in Access Control features are enough for your needs.
Joomla is free open source software. You don't have to pay anybody anything and can download it here free, gratis and for nothing.
Joining Build a Joomla Website will cost you some money - but it will probably be the best investment you could make to help you get to know, and use, the software effectively. And no, I'm not on the staff.
To answer your question, front-end editorship is part of the standard Joomla package. Editing rights to an individual page or pages are something you control and can be granted to either groups or individuals.