Overture allows both internal and external users to view booking information. Internal users are all members of your team and have access to varying degrees of permissions within your Overture account, these are the users you pay subscription fees for. You can also have as many External users as you require these only have access to particular features and information that you can control from within your Overture account.
In relation to the Calendar, these users, when logging to Overture will be able to view the bookings and events that they are part of. In the settings you can setup statuses to control when these bookings appear on external users calendars.
In the screenshot as an example the artist Robert Baker a member of ACME Artists will be able to view this booking since it has the status of Confirmed. In Robert's account along the main menu bar you can see that the options available are limited to just Calendar, Bookings and Docs so this user can only see information relevant to them.
In the sidebar you can see options where the user can select to see their own personal calendar or select the calendar of the artist record they belong to. There is also the option to subscribe to this calendar so external users can import their bookings and events into whatever Calendar apps they might already be using.