If you want to provide access for the portal only to those who are approved by the admin, then navigate to WordPress Admin Panel -> Customer Portal Settings Page. 
Tick the checkbox to enable user approval flow. Now, whenever a portal user registers from portal. They will be added to Contacts module in CRM with pending status.
Now you can apply advance filter on contact’s approval status and get the list of pending contacts for approval. 
Now you can apply advance filter on contacts and get the list of pending contacts for approval. To approve contacts, select such contacts from the filtered contacts’ list view and click on the Actions dropdown. Now, select ‘Approve For Portal’ option from the dropdown. 
This will convert pending CRM Contacts to portal users and a list of imported contacts will be displayed. 
While approval it will also send an email to portal users notifying that they are approved and can access the portal.
Instead of mass contacts approval, you can also navigate to the detail view of any contacts and approve them by changing the approval status from portal user information tab. 
Note: Once the contact is approved, you cannot change the approval status of that contacts.
If verification flow is enabled. They need to verify their account then only they can proceed further for login.