When a customer is invited to join Service Desk, it receives an email with a link for it to visit the portal and set its password.
However, when accessing this link using Internet Explorer 9, only a blank page is displayed.

Steps to reproduce:
Go to a Service Desk's People tab, then to the Customer section
Click the Invite Customers button
Add an email that you can access and click the Send Invites button
Check this email for a "Welcome to Welcome to Service Desk" email, on it there will be a Visit portal button
Open the link on this button using Internet Explorer 9

Workaround:
Use another browser (Firefox, Chrome, Safari), or use a newer version of Internet Explorer.