Connect-PnPOnline $siteUrl -UseWebLogin
$ctx = Get-PnPContext
$web = Get-PnPWeb -Includes WebTemplate, Configuration
$Principal = [Microsoft.SharePoint.Client.Utilities.Utility]::ResolvePrincipal($ctx, $web, "Everyone except external users", "All", "All", $Null, $True)
$ctx.ExecuteQuery()
$everyoneLoginName = $Principal.Value.LoginName
Add-PnPGroupMember -LoginName $everyoneLoginName -Identity $web.AssociatedVisitorGroup
What is the version of the Cmdlet module you are running?
1.2.0
Which operating system/environment are you running PnP PowerShell on?
 Windows
 Linux
 MacOS
 Azure Cloud Shell
 Azure Functions
 Other : please specify
        changed the title
[BUG] Add-PnPGroupMember does not work for "Everyone except external users"
Add-PnPGroupMember does not work for "Everyone except external users"
      Feb 1, 2021
          
Behind the scenes we are not using CSOM anymore in this cmdlet to acquire the correct group, for that reason we require the group to have the ID property loaded before we can add a member to it.
In a more 'powershell' way, this works with the release you have installed:
$user = Get-PnPUser | ?${$_.Title -eq "Everyone except external users"} # but your approach to find the principal will work too
$group = Get-PnPGroup -AssociatedVisitorGroup
Add-PnPGroupMember -Login $user.LoginName -Group $group
We will merge an update today which will actually resolve the id of the group if not specified before we try to add the user to group. This change will be available in release 1.2.11-nightly.