How to add local group to "Remote Desktop Users" group in Windows 7 pro

I am assuming that ToolUsers is a local group on the Windows 7 PC. Local groups cannot be nested within local groups in Windows 7 (or, indeed, any prior version of Windows). It's just a limitation of the product's design.

If you can't handle just putting users in both groups you could attempt to modify local security policy to grant the same user rights to your ToolUsers as is already granted stock to Remote Desktop Users but, personally, I'd just put them in both groups.

If you have an Active Directory domain you can make ToolUsers a domain Global Group or Universal Group and nest it into the Windows 7 computer's local Remote Desktop Users group.

You shouldn't have to maintain users in 2 groups for this. Just add or remove entire Groups from "Allow log on through Remote Desktop Services" Policy config. I actually remove administrators from this configuration as remote admin access increases the high security risks associated with RDP.