"Thank you for your note. It appears that a number of the users you're
attempting to add don't currently have Google Calendar accounts.
Currently, to control potential issue with spamming you can only add a
limited number of users to your calendars at a given time. If you'd
like to add additional users to your existing shared calendars, please
wait 24 hours to do so. You may also want to ask the friends you're
sharing your calendar with to sign up for Google Calendar. If you've
waited 24 hours and continue to experience this issue, please respond
to this message with additional information about the issue you're
experienced. We appreciate your taking the time to report this issue."