You settle the individual lawsuit with the party that started it. For a work with multiple authors, that wouldn't prevent the other copyright holders from suing you. This is one of the reasons GPL projects make lawyers risk-averse...
Generally the way GPL has been in the courts, the suing party won't just take some money and walk away happy, they'll ask for the infringement to end (one way or the other). That would make other copyright holders happy too.
Whoever/whatever owns the copyright to the underlying work(s) would need to sue to get the ball rolling. Current statutory law doesn't directly address copyright ownership with respect to the kinds of collaboration we see in open source, but if there's a "main author" for the code base you would just need them in most cases.
I'm curious, what would that look like in this case (for a GPL violation)?
Do you need to track down every single person who contributed code under GPL and get them to agree?