You can use Portal fior ArcGIS without Acount and be able to search and use public Items. Or you can use Portal for ArcGIS with an Organization account as a User, as a Publisher ou as an Administrator.
The User role defines Search And Use public items, Search And Use private items, Create And Share items.
I created a ligther role based on the Viewer role named "light user". A ligth user can be a member of a group, search and use public items, search and use private items of its organization.
Well it's very close to what you can do if you are not authentified. But the difference is that you consume a "named" user.
Then how to use a login widget in my web apps, and having light user who not consume "named" use ?