If I buy 10 tickets in the SNCF app can I use them for 2 people? Or do we need to have our own app & own tickets?
You would need your own Paris transit ticket app & own set of Paris Metro tickets unfortunately. Once a ticket is validated on a mobile phone/device, that Paris Metro ticket is good for 90 minutes of transfers and validations if "controlled" (ticket checking by transit staff).
The app doesn't have a way of managing multiple validated/active tickets for more than one traveler. This is the principal downside of moving away from the booklets of paper Paris Metro tickets, which could be split amongst multiple travelers for simultaneous use in the past.