As others said---a claimant or claimants sue on behalf of a number of people with similar complaints. The amount in controversy for an individual may be too small to be worth filing a lawsuit (especially in federal court, where the jurisdictional amount req't is $75,000), but when the claims of the class are combined it can be economical to proceed. Once the class of claimants is determined and notice is given, individuals can opt out of the lawsuit if they want to--if they don't, they are bound by the outcome of the class action.

Steve Berman, the plaintiff's counsel in this case, specializes in this sort of litigation. His firm have also been filing a series of individual lawsuits on behalf of KAL passengers in the past few weeks, alleging damages as a result of the fuel-surcharge price-fixing business. I expect that at some point in the near future he will seek to have those consolidated into a class action. Haven't seen any fuel-surcharge filings against BA, however.