If your brand is PRIVATE, they'll see a header with your brand name, description, and any links you have added. Below that will be a login form that only guests, collaborators, and admins will be able to successfully use to see your brand assets.
You may also allow anybody to request access to any PRIVATE Brandfolder, which is turned off by default. Admins may turn off this feature under the "Admin" drop-down in the editor bar by choosing "Manage Notifications".
If your brand is STEALTH, they'll see only a login screen. An example is below: