you can get the list of all subscribed user by following these steps.
At the top, the browser window, click Tasks and then click Subscriptions. All user subscriptions for the current site appear, including information like subscriber name, view name, and delivery schedule.
Select any subscription you want to update. From the Actions menu, select Change Schedule, Change Subject, Change Empty View Mode, or Unsubscribe.(The empty-view option sends subscription emails only when data exists in a view. It's a good choice for high-priority alerts.)
You can also create a Tableau dashboard by connecting your workbook to Tableau Postgres database.
Actually what I'm looking for is to know when a user was added, exact date and that information is not available in that screen or even from the Postgres database, at least I'm not aware of where to get it.