It depends on the settings you have established on the Portal Info page. You can Restrict New Portal Accounts to Members Only and can Restrict Portal Logins to Active Members Only if desired. If these settings are set to YES, then someone who is not already in the database cannot create a portal account.
If you have not restricted access, then anyone can sign up for a portal account. If a matching constituent is found via the email address and name, the information will be linked to an existing constituent. If no match is found, the software will create a new constituent record.