After everyone has set up their iCloud accounts, they can change their Store credentials to your original Apple ID. On an iOS device, open up the Settings app and tap the Store menu. Log out and input the Apple ID. On a Mac, open iTunes and go to Store -> Sign Out, then Store -> Sign In and have them log in with your iCloud account credentials.
Can't find your specific scenario above? Here are a few general tips.
If you have an Apple ID that you want to use for sync and purchases: Convert it to iCloud.
If you want to sync with one ID and purchase with another: Convert one to iCloud; use the other to log into your Store credentials.
If you want to sync with multiple IDs: Hook up your primary account to your device; add a secondary account for calendar and contacts syncing using the Mail, Contacts & Calendars pane in System Preferences on your Mac or in the Settings app on your iOS device.
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