Mac OSX switch to iCloud password login missing
I have a couple of users sharing an iMac (10.11.4). They are logging in with local accounts / passwords.
I have just set them up with iCloud accounts, and I'd like them to log in to the machine with these.
Normally to enable this I would log in as the user, go to System Preferences / Users & Groups and hit the Change Password button, and then click the 'Use iCloud' button. However I'm just getting the regular password change pop-up window, with no option to switch to iCloud.
I have tried signing the user out and in again in System Preferences / iCloud but this hasn't helped.
Any suggestions?
Thanks!