Configuring reporting service for Dynamic Ax 2009

I follow the steps in Ax installation guide on how to install reporting service for Ax.
When I came on setting up application pool, I got this error

The identity of application pool 'AxReportServer' is invalid, so the World Wide Web Publishing Service can not create a worker process to serve the application pool. Therefore, the application pool has been disabled.
This is corrected by assigning Ax proxy account to local IIS_WPG user group.

Later I need to assign ax proxy account to ReportServer and ReportServerTempdb database.