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
Troubleshooting FaceTime and iMessage activation
iOS: About Messages
Set up iMessage
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"?)