Google-apps – Allowing everyone in the domain to edit a Google Calendar

This used to be available, but has been discontinued:

Note: The previous Start Page service was a syndicated version of iGoogle; it allows you to set up a dynamic homepage (one start page layout per domain) for your users that brings together your content, Google services, and the best of the web. Starting on 3 December 2008, the Start Page is no longer included for new organizations that sign up for Google Apps.

(From: http://www.google.com/support/a/bin/answer.py?hl=en&answer=46798)

However, you can now make your own via Google Sites:

As a Google Apps administrator, you can create a Start Page for your users, using the Google Sites Start Page template. This page is your users' homepage, on which they can access their Google services and other content -- such as interactive gadgets and RSS feeds -- when they log in to their Google Apps accounts.their Google Apps accounts.

(From the tutorial: http://www.google.com/support/a/bin/answer.py?answer=179916

Your calendar behaves this way because of the settings in Google Apps. The default config for sharing outside of the organization is showing only free/busy information without the details. Your admin needs to change it to Share all information, but outsiders cannot change calendars or Share all information, and outsiders can change calendars in the settings page of Calendar (simply click Calendar from the admin dashboard).

You can also try setting each of your events as Public when creating them. That's the option in the Privacy control (the very last one) on the event creation page. However, this might not work because of the organization-wide setting described above.