Administrator rights for Computer

I have Admin security group slWorkstationAdmin in domain which is added to
workstations local Administrators group using GPO restricted group utility.

Now I have couple of workstations where (project) users needs Adminstrators
rights. I thought if I add those computer accounts to slWorkstationAdmin
group, then users have rights to install required software only to the
specified computers.

If I remember correctly adding compter account to computer Administrators
group logged users have also Adming rights? This is not working in my case,
but why.....