If a user share contacts with all the users, then all the users will see them. An option is needed to ignore all shared contacts, or at least to filter them out.
If a use tries to delete a shared contact, or move it to another group, nothing happens, and no error is displayed.
If a user has a contact named “John Doe” and another user shares a contact with the same name, searching by name to send an SMS will find only one, and that maybe the wrong one. User contacts should have priority.