Create local user as part of Administrator group on Windows 10 - powershell

Question

Setup:
We are all in domain environment. Active Directory is Windows Server 2012 R2
Client Workstations are mix of Windows 10 versions (1703 and 1709)
Need to create user "Servis" on all workstation and place it to local Administrator group. I will input a text file with the host names.
This is the script im trying to make it work, but no success.
The user is created, but user is not added to local admin group.
This is the error i get :
Error creating Service23 on WinNT://WS-TEST: The following exception occurred while retrieving member "add": "The network path was not found.
$computers = Get-Content -path C:\Scripts\CreateLocalUser\New\Computers.txt
$username = "Servis"
$password = "P4$$w0rd!##"
Foreach ($computer in $computers) {
$users = $null
$computer = [ADSI]"WinNT://$computername"
Try {
$users = $computer.psbase.children | select -expand name
if ($users -like $username) {
Write-Host "$username already exists" -ForegroundColor Green
}
Else {
$user_obj = $computer.Create("user", "$username")
$user_obj.SetPassword($password)
$user_obj.SetInfo()
$user_obj.Put("description", "$username")
$user_obj.SetInfo()
$user_obj.psbase.invokeset("AccountDisabled", "False")
$user_obj.SetInfo()
$users = $computer.psbase.children | select -expand name
if ($users -like $username) {
Write-Host "$username has been created on $($computer.name)"
$group = [ADSI]("WinNT://" + $computername + "/administrators,group")
$group.add("WinNT://" + $computername + "/" + $username + ",user")
}
Else {
Write-Host "$username has not been created on $($computer.name)"
}
}
}
Catch {
Write-Host "Error creating $username on $($computer.path): $($Error[0].Exception.Message)"
}
}

Answer 1

Once you've defined your credential correctly with a password in a SecureString format creating a local admin is a 2 step process:
Create a user
Add that user to the Local Administrators group
These few lines are the powershell code needed:
$username = "Servis"
$password = ConvertTo-SecureString -AsPlainText "P4$$w0rd!##" -Force
New-LocalUser "$username" -Password $password -FullName "$username" -Description "Local admin $username"
Add-LocalGroupMember -Group "Administrators" -Member "$username"
If you want make you script more robust you should consider to create some Pester tests and add these checks to your script:
Validate if your file exists
Validate the content of the file (if is an IP or a FQDN with a regex)
Test if the host is reachable (e.g. Test-Connection -computername $computer -count 1 -quiet) before connecting
Using invoke-command with computername and argumentlist (username, password)
Check if the user was created successfully and is a member of the administrator group if not throwing an error
Write a log file to keep trace of all the tasks completed (by who, when on what host and so on).
I wrote a blog post on www.scriptinglibrary.com on few better ways of completing this task if you're interested.