When setting up your portal it is very useful to preview the portal as some of your users. This allows you to test that permissions are working as you expect and to see exactly what your users will see.
You can impersonate a user from the Users page. Use the Quick settings menu and pick Impersonate Users or go to Setup Home and select Users.
Find the user you wish to impersonate. There are filters and a search box to help locating the user you want to impersonate.
Click the Impersonate button next to the user of choice.
Notice that now you see the users name in the top right corner of the portal, you are logged in as them.
When you have finished testing as the user you can stop impersonating.
Select the users name in the top right hand corner and then select Stop Impersonating. You will be returned to the users page.