if (!($cred)) {
    $cred = Get-Credential -Message "Enter your Domain Admin credentials"
}

$CSVMembers = Import-Csv 'C:\users\eeckert\Downloads\GPO Manual.csv'
$GroupName = 'GPO_Radiation Manual'


foreach ($name in $csvmembers) {
    $Filter = "Name -eq '" + $name.name +"'"
    try {
        $ADObject = Get-ADUser -Filter $Filter 
        Add-ADGroupMember $GroupName -Members $ADObject -Credential $cred
    }
    catch {
        $message = $name.name + ' not found. Add to the group manually'

    }
}

# now get current group members, and remove them if not in the list.