If the message "Your survey link is invalid" is shown when a contact or lead clicks on a survey link or the survey hangs when submitted, check the sharing setting for that object is set to at least 'Read Only'.
In case Contacts and/or Accounts or Leads must remain with sharing set to 'Private' follow the workaround below. Another option is to send the invites through the Email collector (instead of Contact or Lead) by exporting their email addresses and then importing the addresses to the collector in a csv file.
1. Sharing rule needed when sharing setting is set to private
2. Click 'Setup'
3. Click 'Manage Users'
4. Click 'Public Groups'
5. Click 'New'
6. Add Site Guest User to the public group
7. Click 'Security Controls'
8. Click 'Sharing Settings'
9. Scroll to Contact Sharing Rules and click 'New' (Please note that if Contact default access is 'Controlled by Parent' the sharing rule has to be created on the Account object).
10. Click 'Save'