Firstly, you might find this page a good read: https://wiki.roll20.net/Using_Roll20_while_Playing_In-Person
Basically, you want a composite revealed Advanced Fog of War, where anything seen by any player is seen by all players. The only way to do that automatically is with an API script, which requires a game created by an active Pro subscriber. Observer is designed with this use case in mind, there are some other options on that page above. To do this manually, you can either give control of all player tokens to the user logged into the session for the table, or you can use the Fog of War reveal tools to show areas.