It sounds like you need a content management system that can integrate with your existing website. I would recommend checking out our Top 10 Web Content Management report, or searching the Product Finder for web content management.
We actually have a post about these 3 popular CRM. You can find it right here http://www.business-software.com/blog/comparing-the-big-three-cms/
For a quick summary:
Wordpress: super easy to use. A lot of things are pre-made for you already, you just have to play around to get a hang out it.
Joomla: Still super easy to ...
It really depends on what you're looking for. There are so many plugins for different features. Just to name a few, there is Role Manager which allows you to control roles and access different authors on your blog. There is Adminimize which gives you the power to assign rights to ...