I am reading about Paris and I saw that you can buy a pack of 10 tickets.
Are these only usable by one person, or they actually sell 10 separate tickets that you can share with others?
The carnet comes as 10 separate tickets and nothing stops you from sharing with others.
Indeed you can. I'd extend this to other French cities as well: I have lived in Lyon, Clermont-Ferrand, Mulhouse and (currently) Strasbourg, and in each of these cities public transportation tickets can be bought by 10 and shared.
Be sure to buy an actual pack (fr. carnet) of paper tickets if you plan to use them for several simultaneous passengers. If you buy electronic ticket batch (loaded on a contactless card) or a single multi-ride ticket, you won't be able to validate it several times in a row. This can be tricky to recognize, as some tickets will show you green light when validated several times, but only one ride will be deduced, and all passengers except one will be fined if controlled.
If you're traveling with family or friends, I advise you to check group ticket prices (usually called DUO, TRIO, FAMILLE etc.), they are often more advantageous than 10-ticket batches, provided that your group fulfills the requirements of the ticket.
Yes! They're not labelled, are separate, and work independently.
Source: Bought in bulk when in Paris with a large group. Worked out just fine. Also saved us money.
Second source: Some information about the tickets, prices and options: Paris by Train - Paris Metro Tickets.