MyAthens is a basic user portal that any user account can sign into to find a list of allocated resources, view information about their organisation, and contact their administrator.
There are some options for customising it including whether to show a page of information first, or go straight to the list of available resources.
MyAthens is available at: https://my.openathens.net and at https://my.openathens.net/?entityID=YOURENTITYID. The second of these addresses will take the user to your version of the sign-in page and is the one you will prefer if you are using local accounts. You can look up your entityID on the organisations page ()
The list of resources the user sees is controlled by what you have placed in the permission sets that are associated with the user's account. If you change these resources, the user's access rights are updated immediately but they won't see the change in MyAthens until the next time they sign in.
It is MyAthens that the hidden from users flag on resources is used in.
The 'contact your administrator' function tries to use the public contact email for the domain administrator as the most likely suitable support contact. If this email is not available it will use the email address of the administrator that the user account sits directly under.