Error description not available

What did the error message say?

Using FaceTime http://support.apple.com/kb/ht4319

Troubleshooting FaceTime http://support.apple.com/kb/TS3367

The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting

http://tinyurl.com/a7odey8

Troubleshooting FaceTime and iMessage activation

http://support.apple.com/kb/TS4268

iOS: About Messages

http://support.apple.com/kb/HT3529

Set up iMessage

http://www.apple.com/ca/ios/messages/

Troubleshooting Messages

http://support.apple.com/kb/TS2755

 Cheers, Tom 😉

Took me a while to figure this out, but I'll bet the address you're trying to add is or was registered as an Apple ID. You can have all sorts of phone numbers and email addresses in the "receive at" lists for iMessage and FaceTime, but you CANNOT have two different Apple ID's among them. I found this in a blog somewhere; sure wish Apple would make this clearer, especially in the error message itself!!! (How about "This address is already in use an as Apple ID and cannot be added"?)

Agree - this message is meaningless and no documentation for it that I could find - except here.

But changing from the family shared iCloud account to my personal ID worked.