Powershell get-aduser not showing all results

There are a few ways to go about this:

Get-ADUser -LDAPFilter "(!GivenName=*)" | ft samAccountName

Get-ADUser -Filter * | Where {$_.GivenName -eq $Null -OR $_.Surname -eq $Null -OR $_.Name -eq $Null} | select samAccountName

That should work for you.

Added a second command that will check if any of the "name" fields are blank.

Add as the first line of your CSV file a row containing the column headers for your file. You then use those headers as the property names when iterating through the file.

So your file should look something like:

Firstname,Lastname
Joe,Schmoe
John,Doe
Jane,Doe

Then your code should work as is.

Also, the filter seems not to like $Name.lastname. I'm not sure exactly why, but even property expansion is not doing the trick. If you collapse this into a single string variable it work:

$names = Import-CSV C:\path\to.csv
ForEach ($Name in $Names)
{
    $nameFilter = $Name.Firstname
    Get-ADUser -Filter { GivenName -like $nameFilter }
}