You can find all the information on the Friends Day Pass for Youth here: Friends Day Pass for Youth | SBB
You need one ticket for all. Upon purchase, all persons in your group must be declared, hence must be registered with surname / name and date. The ticket cost is CHF 80, no matter if you use the ticket alone or as a party of 2, 3, 4 people.
Does that answer your question? Do not hesitate to contact us if you have more queries.