By default, the Users list view is filtered - showing Internal Users only. When you remove that filter, you'll see all active Users:

Alternatively, you can add the Portal Users filter:

For clarification:

Internal Users are users with access to the backoffice, backend, admin area or whatever you'd like to call it. In case you're running Odoo Enterprise these are the Users you are paying the subscription fee for.

Portal Users are users without that access. These users can see frontend portal only (only their Sale Orders, Invoices etc.).