From AgileApps Support Wiki
Typically, contacts are invited to use the Support Portal when Self Registration is turned off. (If it's turned on, you only need to send them a link, or put the link on your website.)
To invite a contact to use the Service Portal:
- This choice appears only when the contact is not already registered as a Service Portal user
- When clicked, a portal username and password are created. The Service Portal Invitation email template is then used to send the portal link to your customer, along with the username and password.