In version 3.13, single sign in enhancements were introduced, allowing contacts to login to the company portal page and be redirected from there to an individualized default portal. For more information, see Online Portals - Portal Login.
If you have a Default Portal selected at the top of the Online Configuration page, when you login to the company portal page as an Admin you will be redirected to that default portal and only be able to see the contacts assigned to that portal.
If a contact is assigned to a different portal, they will be redirected to that portal if it has been set as their default portal in their individualized contact settings or if that is the only portal they have access to. A contact will only be visible on their specified portals. For more information on assigning portals to a contact, see Contacts - Online Ordering.
In order for you to go to their default portal, you will need to include the portal ID in your URL when navigating there.
When you login to their unique portal, their profile should be visible on the portal and you should be able to assume their identity or that of any contacts for that portal.