Adding security groups to Reporting services security doesn't take effect until IIS is restarted

If you grant Reporting services permissions (via https://servername/Reports  | folder properties | security) to Windows security groups, you may find out that people belonging to those groups don't immediately get the expected access rights.

For performance reasons IIS caches authorization tokens. For more details on how to configure it, see the following KB article:

https://support.microsoft.com/kb/q152526/