Get-AzADGuestLastLogin.ps1

<#
.DESCRIPTION
Get last login of Guest user in Azure AD
Adapted from https://cloudtech.nu/2020/05/03/export-azure-ad-last-logon-with-powershell-graph-api/#comments
.NOTES
Requries an Azure App registered with Add AuditLog.Read.All, Directory.Read.All, User.Read.All permissions.
Set clientID, tenantName, clientSecret in $env: vars.
#>

$clientID = $env:clientID
$tenantName = $env:tenantName
$clientSecret = $env:clientSecret

$ReqTokenBody = @{
    Grant_Type    = "client_credentials"
    Scope         = "https://graph.microsoft.com/.default"
    client_Id     = $env:clientID
    Client_Secret = $env:clientSecret
}

$TokenResponse = Invoke-RestMethod -Uri "https://login.microsoftonline.com/$TenantName/oauth2/v2.0/token" -Method POST -Body $ReqTokenBody
$uri = 'https://graph.microsoft.com/beta/users?$filter=UserType eq ''Guest''&$select=displayName,userPrincipalName,signInActivity'

$data = while (-not [string]::IsNullOrEmpty($uri)) {
    $apiCall = try {
        Invoke-RestMethod -Headers @{Authorization = "Bearer $($Tokenresponse.access_token)" } -Uri $uri -Method Get
    }
    catch {
        $_.ErrorDetails.Message | ConvertFrom-Json
    }
    $uri = $null
    if ($apiCall) {
        $uri = $apiCall.'@odata.nextLink'
        $apiCall
    }
}

$result = ($data | select-object Value).Value
$result | Select-Object DisplayName, UserPrincipalName, @{n = "LastLoginDate"; e = { $_.signInActivity.lastSignInDateTime } }