can't share iCloud document
So I put a document in my iCloud drive, and ask to share it. I type in an e-mail address (someone who it recognizes as being in my contact list), and it comes back with "People couldn't be added. There may be a problem with your server. Please try again later".
Except I just before that shared the document with myself at another e-mail address, so a problem with my server seems unlikely. What's going on?
iMac, OS X 10.11