Google Docs and Calendar – Unauthenticated Access Issues

Your Google Apps admin has not allowed users to receive documents from outside your domain.

One of the sharing options you can set for Docs & Drive within a Google Apps account is weither or not users can share documents to people outside the organisation. One sub-option of this is that they can separately allow or deny users from receiving outside documents

As the administrator, you determine whether users can share their Google Docs (documents, presentations, spreadsheets, and drawings) outside your organization, whether they can access docs created outside your organization, and the default visibility level for new docs.

If you are the administrator of the domain you can change the settings via:

1. Sign in to the Google Apps administrator control panel.
2. Click the Settings tab and then select Drive and Docs in the left column.
3. In the Sharing options section, choose whether users can share docs outside your organization.
4. Hit Save

I'm not sure if this is on or off by default but just ask your admin if they can allow outside sharing. Worst case is they will say no because of X.

The private link can be used to share the calendar with non-Google Calendar users (e.g. Outlook or Yahoo Mail). It appears that Google does extra checking and doesn't allow a private link to be added if you weren't granted access (with Share with specific people setting).

If you mirror the file to a new URL or simply use an URL shortener (like goo.gl) you can import it into Google Calendar with no problem, bypassing the checks.

Other than that you have correctly outlined the options available. The Calendar ID is the same for the private and public links.