Sitefinity is using standard ASP.NET membership, which means that everything that applies to membership in ASP.NET also applies in Sitefinity. You can find more information about ASP.NET membership here:
Depending on the specific requirements you may decided to give master permissions to the CMS itself or you can build several pages in Sitefinity that would be member aware (only logged in users could access them and/or only users in specific roles). If you take second approach, these protected pages would have controls that would take advantage of Sitefinity API and master users could add new users and permissions.
In any case, you can achieve the desired functionality with Sitefinity. Once you start development, feel free to ask us more detailed questions and we'll assist you with those as well.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.