I'm also lost here. I get a few invites per week on a few mail accounts. I've added thosemail addresses to Google in the past. You can't create a calendar account for an e-mail address that already registered as a secondary account. The suggested work-around doesn't work: even after deleting the mail accounts as secondary accounts I had to wait for days or endlessly before the addresses were no longer 'already registered'.

Can't ask everyone to invite on another mail account all the time.

The same goes for Google docs, by the way.