First of all, let me welcome you to the Joomla forums.

You are running up against the limits of Joomla's user access, as you have found.

There are a number of extensions that address this in different ways, and you will find a list of them here:

http://extensions.joomla.org/index.php?option=com_mtree&task=listcats&cat_id=1782&Itemid=35

I would recommend that you take a close look at JACL Plus and JUGA:

JACL Plus:  http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,387/Itemid,35/

JUGA: http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,2587/Itemid,35/

There are several others in the category, and you may find something else that will meet your specific needs better.

I hope that helps.