Currently, our built-in security features are more concentrated in the administrative part of Sitefinity, not in the public part. The security model that you propose is not simple. You can use Sitefinity as a basis or a framework, and develop custom controls and modules that integrate with it to implement permissions and security roles for the public part. So, generally, it is possible with some development work on the public permissions.
We are working hard to improve the security roles in the public part and believe that many of the tasks you would target will be done in some of our next releases.
Feel free to contact us if you would like to know anything else about Sitefinity.
the Telerik team