Hi,
assigning users to whitelists should be possible – I tested this on 4.6.6 and on 4.8 and it worked in both cases.
What exactly does not work for you? Are there any error notifications? Have you tried disabling other plugins, in case there is a confict?
You probably know this, but, note that not all users appear in the whitelist GUI, because not all users can be assigned for logical reasons: i.e. you can’t assign administrators (because they could un-assign themselves anyway), or users who don’t have the edit-others-pages capability (because then there is nothing to hide from them).
C.