I will be using a domain windows account(maintained in Active Directory) as Application pool identity in my web server. So i want the account to be a member of IIS_W
I'm not sure about adding via Active Directory as a blanket fix.
We used this method for IIS 6:
Add User to IIS_WPG group: Go to Computer Management on the box hosting the site, Local Users and Groups > Groups, right-click IIS_WPG > Add to Group, then add the windows account, Apply, OK.
Then allow account to run as service: Admin Tools > Local Security Policy > Local Policies > User Rights Assignment > Log on as a service Properties, Add user, Apply, OK
Set App Pool Identity: IIS > App Pool Properties > Identity tab, set as configurable and input user, Apply, OK.
Then reboot box.
Good resource: Steps for setting up a custom identity of an IIS 6 application pool