Address Book sharing problems

Recently, we've had problems with the Address Book sharing function. I'm hoping someone on this forum has some troubleshooting ideas they can share.

Here's the background: I had given my assistant the ability to share (and edit) my address book, a tremendously handy feature for us in our small office. But it has now stopped working. My address book disappeared from the sidebar of his Address Book application. We have tried to initiate sharing again from my end and from his. Neither works.

We have also reset the .Mac sync data for the Address Book, and then tried to initiate the sharing again. To no avail.

And finally, we have both trashed our address book preferences files (com.apple.addressbook.plist and com.apple.addressbook.adb.plist). That didn't help either. If anyone has any ideas about the cause of this problem, I'd appreciate some input.

Thanks,
Chuck