Question

I’m working on a script that will check if a user is an Administrator and then if they’re not it’ll add them on the spot, logoff, and then I can log back on to test. This is the part I’m running into problems:

$Cred = Get-Credential ("$env:COMPUTERNAME\Administrator")
$Group = [ADSI]("WinNT://"+$env:COMPUTERNAME+"/Administrators,Group")
$User = $env:USERNAME
$Domain = $env:USERDOMAIN
Invoke-Command -Computername localhost -Cred $Cred -ScriptBlock {$Group.add("WinNT://$Domain/$User,user")} 

Everytime I run this I get the following error:

You cannot call a method on a null-valued expression.

Any ideas what I can do to fix this issue?

Working solution:

$Cred = Get-Credential ("$env:COMPUTERNAME\Administrator")
$User = $env:USERNAME
$Domain = $env:USERDOMAIN
Invoke-Command -Computername localhost -Cred $Cred -ScriptBlock {
    param ($User, $Domain, $ComputerName)
    $Group = [ADSI]("WinNT://$ComputerName/Administrators,Group")
    $Group.add("WinNT://$Domain/$User,user")
} -ArgumentList $User, $Domain, $ENV:COMPUTERNAME

Answer 1

Invoke-Command will know nothing about $Group variable, that’s reason why it does not work like that. You need to pass your variables to scriptblock using -ArgumentList parameter.

Also: I would rather define things like $Group inside this scriptblock:

$Cred = Get-Credential ("$env:COMPUTERNAME\Administrator")
$User = $env:USERNAME
$Domain = $env:USERDOMAIN
Invoke-Command -Computername localhost -Cred $Cred -ScriptBlock {
    param ($User, $Domain, $ComputerName)
    $Group = [ADSI]("WinNT://$ComputerName/Administrators,Group")
    $Group.add("WinNT://$Domain/$User,user")
} -ArgumentList $User, $Domain, $ENV:COMPUTERNAME

HTH

Bartek

PS: just one question: why not doing it simple way, with:

net localgroup administrators domain\user /add