If anybody has a large Joomla 3.x site, and is wanting an easy way to manage it’s access control lists, I recommend the ACL Manager by Sander Potjer. It makes it easy to figure out which group or user has access to any resource, including those on the administrator side of the site. Go check it out!

Site: https://www.aclmanager.net
Joomla Extensions Directory: http://extensions.joomla.org/extensions/extension/access-a-security/site-access/acl-manager

(I am not affiliated with either Sander Potjer, or Open Source Maters, the people that maintain Joomla)