If you have came across this TechNet link, you will find that the SP 2010 setup user account should be present in IIS_WPG group. We faced some issue for which we were investigating the account permissions and their group memberships.
On our servers we found that there is no IIS_WPG group. Initially I thought this can be the cause of issue. But I also noted that there is IIS_IUSRS group. I never paid attention to this group or never came across any such group in the context of Web development.
I googled for IIS_WPG and IIS_IUSRS keywords. I found a good link explaining the difference of these group. IIS_WPG is replaced by IIS_IUSRS group in IIS 7. Our servers are Window 2008 based and so our systems have the IIS 7.
IIS_WPG group is specific to earlier versions of IIS.
So if you find similar discrepancy in SharePoint systems, do not panic. Check for availability of IIS_IUSRS group.