One problem: We are using Clear (https://itunes.apple.com/gb/app/clear/id504544917?mt=12) which stores data in iCloud's Documents & Data, and would love to access ALL family members' Clear lists on ALL Macs. How is this possible when each member has his/her own ID?
Same problem for accessing Pages documents or Preview files (in the iCloud) from all Macs.
We're afraid we'll all have to use the same ID for iCloud then...?
There is (or may be) a way around this.
You can log into more than one Apple ID for iCloud at once, you can have a primary account and any number of secondary accounts, secondary accounts cannot be used for document & data syncing, find my mac, back to my mac or photostream.
Providing you don't mind sharing documents and photostream, this is what you would do.... All use the same ID for a primary account but turn off mail, contacts, calendars and the other iCloud features you don't wish to share. Add a unique Apple ID for each person as a secondary account and turn on mail, contacts calendars etc.
There would need to be some trust for this system, since you could go into your settings and turn someonelses mail for example and see it on your device.