Needed Api Permission for Service Principal to invoke Get-AzureADUser

This week I extended our custom B2B invitation handling build as Azure Automation Runbook to check upfront whether guest account is not present yet. The way to check is by quering for potential presence of requested Guest account via the UPN:

if ($guestAccRequests.length -gt 0) {
    # Get the Service Principal connection details for the Connection name
    $servicePrincipalConnection = Get-AutomationConnection -Name $servicePrincipalConnName   

    # Logging in to Azure AD with Service Principal
    Connect-AzureAD -TenantId $servicePrincipalConnection.TenantId -ApplicationId $servicePrincipalConnection.ApplicationId -CertificateThumbprint $servicePrincipalConnection.CertificateThumbprint

    ...

            # 1. Check whether already present as guest account
            $guestUpn = ConvertExternalEmailToUpn -guestEmail $guestEmail
            $existingAccount =  Get-AzureADUser -Filter "userPrincipalName eq '$guestUpn'" | Select UserType, UserState
            if ($existingAccount) {
                ... 
            if ($continueToInvite) {
                try {
                    $inviteResult = New-AzureADMSInvitation -InvitedUserEmailAddress $guestEmail -InvitedUserDisplayName $guestDisplayNameWithOrg -InvitedUserMessageInfo $messageInfo -SendInvitationMessage $False -InviteRedirectUrl "http://www.wvstrien.com" -ErrorAction Stop
                    $inviteurl = $inviteResult.InviteRedeemUrl

The PowerShell coding part was quickly up-and-running. More time-consuming was identifying the proper Api Permission to grant to the Service Connection that connects into Azure AD. We already assigned the 'User.Invite.All' Api Permission from the Microsoft Graph Api, and one would expect that also permission for Get-AzureADUser is within the Graph Api permissions. However, granting the likely permissions 'User.Read.All', 'Directory.Read.All', 'Domain.Read.All'; none of these authorizes the Service Connection.

The information provided by Microsoft in identifying the needed Api Permission is not very good, or at least not easy to find. The StackOverflow post Graph API - Insufficient privileges to complete the operation put me on the right track. Turns out that for this cmdlet you still need to assign permission from the deprecated Azure Active Directory Api.