I just added "my Google account" to one of my family separate user accounts, with their own Google account (one that has no payment method assigned), on the Nexus 10. When you do this make sure you tell it NOT to sync with your existing account. This way, that family member or other user, can use already purchased apps on the same tablet (Nexus 10 for me). What is interesting, it appears that in fact for the free apps, they do install separately in each user account. I've uninstalled some free apps on my user account, and they still exist in the other users accounts? Also remember that that other user can get rid of or change your added Google account in terms of syncing. If the "other Google account" has no payment option, then that user will not be able to install paid apps.