How do you send an email to all the users (Clients)

There are times where it's necessary to send an email to a number of registered users (Clients) such as holiday opening times, price changes, promotions or if a Provider is unexpectedly off work due to illness.

Is this something that can be done within the plug-in or is there another method that can be used? I can't see anywhere in the plug-in admin where user (Clients) data is stored.