Some customers using Pages, Numbers, Keynote, or the Apple Notes app are currently unable to share new files or add new people to shared files due to an ongoing outage affecting some iCloud functionality.
The problem started at 4:00 a.m Pacific Time this morning, and is still affecting customers, according to Apple's System Status page.
The Notes and iWork for iCloud apps appear to be the only apps that are experiencing an outage at this time. It's not clear when the problem will be resolved, but we'll update this post when it's fixed.
Update: According to Apple's System Status page, this problem has been resolved and iCloud sharing is now working normally for iWork and Notes.