For inviting existing contact records as user of the portal we can follow the below steps.
Open an existing contact record and click on Create Invitation button
This creates the invitation record along with the invitation code.
Select and run the Send Invitation workflow.
The steps for the workflow are as below.
The contact\user receives the link for the confirmation
Clicking on the link opens the portal wherein user\contact can redeem the Invitation Code and Register.
User can set up username and password during registration.
Once registered user can access the portal and update the profile further and can also change the password, email etc.
The Contact record in CRM gets updated accordingly.
Hope it helps..