This is a great question, and one that I get a lot. Sharing an Apple ID for purchased material is great, but it causes issues with sharing contacts if you don’t set it up correctly.
To understand what’s happening, think of the information on your phone as two different categories:
1. data you purchase/download from Apple (Apps, Music…), and
2. personal data (Contacts, Calendars…).
You should share one Apple ID for all iTunes and App Store purchases. This will allow you to share purchases across your devices. Go to the Settings -> iTunes & App Stores and sign in with the shared Apple ID. To avoid all downloads going to all your devices, turn off Automatic Downloads in Settings -> iTunes & App Stores. To keep your personal data separate, you should setup separate Apple IDs for each user. Go to Settings -> iCloud and sign in to your own personal Apple ID on your device. Each other user should sign in to their Apple ID on their device. This will keep your personal data separate. This article provides a detailed description on sharing iTunes and App Store purchases on multiple devices.