Before you enable a portal be sure to choose which standards the external users may dispatch in. Then after a portal is enabled, make sure to add your approved user(s).
Navigate to Accounts
Click on your desired account you wish to add an external user to that portal
Go to the ContentPortal tab
Under GeneralSettings look for the Standards section and choose all that apply
Then under the Activation section, click the checkbox for EnableContentPortal and hit Save.
From here make sure to add your Approved users in order to grant them access the portal.