User system integrated with joomla

Hi all, we need a user system for a website that do the following: 1) A user login into the site 2) The system check what privileges the user have 3) The user can view only some areas of the site, depending from point 2 Pretty simple to do in php + mysql, but the most important thing is that the system must be integrated with a joomla site. So it would be a module or component, or just a php script, don't mind, but it can be easily integrated with joomla.