Person unique identification
This isn't really a database administration question, but one about your business rules (real or imaginary). If you look at the internets, the most common way to uniquely identify users is by their email addresses or usernames (handles, aliases etc.).
Storing national IDs, which constitute sensitive private information, is a bad idea, as  your application then will need to comply with all sorts of regulations that apply to the processors of SPI.
Allowing users to view other users' data is an even worse idea, as that would be a direct violation of various privacy laws.