Allow Domain Users to Install Software locally on their computers

[CommodoreS]

Using a Windows 2008 R2 server I would like to allow users to be able to Install Software locally on their computers, by using a GPO Policy.

I have tried creating a GPO called "Local Admin Rights" and linking this to the OU which contains the machines. The settings are:
Computer Config>Policies>Windows Settings>Security Settings>Restricted Groups
Group Name: Domain\Local Admin Rights (Domain is actual domain name)
This group is a member of: Administrators (I added builtin\administrators but when you go back into the GPO it only shows Administrators and the builtin\ part is missing)

I have also added the Group "Local Admin Rights" to the users but this is not working. Users still cannot install software locally. I am wondering if there may be another setting somewhere that I am missing?

[tmcd35]

I've used GPO Preferences to do this.

Computer/User -> Preferences -> Control Panel Settings -> Local Users and Groups

I've added domain\staff_group to the local Administrators (built-in) group. Seems to work for me.

[CommodoreS]

I will give that a go.
Here is a more detailed guide of what I have tried:
How to make Domain User as a Local Administrator for all PCs - TechNet Articles - United States (English) - TechNet Wiki