Shared apps are stored only once on the device (with just the data and settings part being specific for each user), so whenever one user decides to update a shared app, it gets updated for all users (Source: Heise.DE). As usual with some of the real important details (see: Google Backup), there are no details (or even manuals) describing exactly how it works. However, the announcement page holds:
Users can install and uninstall apps at any time in their own environments. To save storage space, Google Play downloads an APK only if it's not already installed by another user on the device. If the app is already installed, Google Play records the new user's installation in the usual way but doesn't download another copy of the app. Multiple users can run the same copy of an APK because the system creates a new instance for each user, including a user-specific data directory.
It's left open at what point this applies. But if it applies at the initial installation, it will certainly be the same with updates (as the Heise article linked confirms; to those who don't know Heise: it's one of the leading tech magazines in Germany, and well trusted).