Outside of the Metro station Acropoli (Exit "Makriyianni/ Dionysiou Areopagitou"). Google Maps link of the meeting point: https://goo.gl/maps/YDYZDL39PMSNAKWD6. Please arrive at the meeting point, 10 minutes before the tour starts. Your guide carries a sign that says "Little Owl Tours".
The Acropolis Tour ends on top of the Acropolis Hill.