But what if I share my Apple ID? If multiple people use your Apple ID (say, if you have one central account for your, your spouse’s, and your children’s purchases), you can still use this ID among you for purchases, but each person will have to create a separate iCloud account for syncing and backup.
For example, if you have an Apple ID (or MobileMe account) you use between you, your significant other, and your daughter for purchases, but they'd like to take advantage of iCloud's sync and storage features, you'll want to do the following: Have each family member sign up for an iCloud account for their backups, mail, etc; then, after setting up iCloud on their iOS devices, go to the Store preference pane within the Settings app, sign out of the iCloud account (which will populate there automatically after you sign up), then sign into the original Apple ID you use for purchases.
If the Apple ID your family uses for purchases is also your primary account for syncing (if it's a MobileMe account, for example), you personally can use it for iCloud; but your family members will need to create separate iCloud accounts so that they don't accidentally end up syncing with your information.