a ning developer’s thread discusses this. currently nothing from ning yet, but there are hacks.
march 25:
i just realized i should update this. i added a calendar to a ning site of mine by simply adding a new page, and then sticking a google calendar in the page. very simple. doesn’t have within-ning functionality (meaning, not entirely contained within ning), but the most simple solution i could find for the people i designed the site for (a sports team). i like this specific solution because i was able to add a link, along with the calendar, on the page to the calendar’s .ics file, so that anybody on the team, as well as fans, could download the team’s schedule and import it into their own calendar, whether it be ical, google calendar, microsoft outlook, etc.
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Several people have had good results publishing Spongecell calendars to Ning. Andrew just posted a very simple walk through – http://getsatisfaction.com/spongecell/topics/add_a_calendar_to_ning
Marc