Probably everybody is familiar with the calendar event invite features?
If you use Microsoft Windows (with Outlook) or Mac OSX (with iCal) or just a basic browser with a GMail account and you use the calendar functionality, you would have realized, that every time you get invited or invite someone to an event, you get a basic email, that adds the recipient to the calendar event.
I had to write an invitation tool that was cross compatible between different platforms and integrated with SugarCRM for a customer of InsightfulCRM (Australian SugarCRM Gold Partner) where I work.