The guide waits with a name sign just outside the ticket gate of the JR Marunouchi North Exit of Tokyo Station. This is the JR exit, not the metro exit. The start time is 18.00 (6 PM). Rarely the start time becomes 19.00 (7 PM), if so, you will be notified by Viator message and via WhatsApp.
The tour ends in Asakusa, the guide can either drop you off in a restaurant or bar in the neighborhood, or the guide can drop you off at the entrance of the metro station.