Apologies in advance for what should be a simple question along with I understand the full answer is likely to be "ask the airline"... but right now we don't know which airline will be used!

What happens if you book a group of 4 on a single flight booking and 1 of the 4 doesn't turn up? We're considering travel (that will potentially have connections, and is definitely a round trip!) and one of the party might not make it (additionally, they're a minor so can't solo book).

Thanks in advance!