Recently we had a question how to give users access to SSRS and report builder? The problem was an "Access denied" for users not in admin groups. Therefore we had to review the Security settings for Report Services.
We found out that Report Builder can be accessed through an URL or from Report Manager, a component of Reporting Services that enables users to publish and view reports on a report server. Users need assigned permission to access Report Builder.
In the role-based security model that is implemented for Reporting Services, users who are assigned to the Content Manager role can create and edit reports in Report Builder. Local administrators are automatically assigned to this role. If you want other users to be able to work with a Report Builder report, you must create a role assignment for them that includes the default role "Report Builder."
Alternatively, you can create a custom role definition. As long as the customized role includes the "Consume reports" task, users who are assigned to that role will have sufficient permission to create and modify reports using Report Builder.