You can easily reach the meeting point using public transportation (look for the "Piazza Risorgimento" stop). Once you arrive there, walk along Via di Porta Angelica and turn left just before the arch leading into St. Peter’s Square. Please, arrive 30 minutes early at the meeting point.
At the end of the tour, you will re-enter St. Peter’s Square, where the group will disband and guests will be free to continue exploring the city on their own.