We only allow users to sign up for a portal account if the email address being used to sign up is associated to an existing contact with portal access.

If you need portal access to an account, the owner of that account will need to add you as an authorized contact and grant it portal access.

When a user signs up for service through the Order Form the primary contact is always granted portal access by default. During the sign up process they are able to add additional contacts and grant them portal access.