Ah, I see your point. But those notices are where a plaintiff has already filed suit, a class has already been certified, and a settlement offered. You are being notified as a possible party to the settlement of your right to accept the settlement terms or opt out and bring a lawsuit of your own.

In keeping with the Star Wars example, suppose David Prowse sued on behalf of himself and all other costume-actors in Return of the Jedi - by 'costume actors' I mean people who appear on the screen but don't speak any lines. Darth Vader's lines are all spoken by James Earl Jones because David Prowse has a goofy accent that's not appropriate to the character; the Screen Actor's Guild mandates quite different royalty payments for actors who talk as against actors who merely appear. If a court agrees that 'costumed actors' constitutes a viable class of identifiable persons who are similarly situated (they probably would; they're listed in the credits, and many actors appear without having any lines to speak) then it may certify a class on the basis that it's more reasonable to wrap multiple plaintiff's claims into a single suit. A lot would depend on whether the court considered the suit to have any merit, which would require looking at industry employment agreements of the time. I don't know much about labor law so I couldn't tell you how likely that is. My gut feeling is 'not very' because these compensation practices are widespread. Anyway, say Prowse does a good enough case that he either wins, or Lucasfilm fears that defending the case will cost more than paying out and offers a settlement. Then everyone who played an Ewok/Stormtrooper/whatever but didn't have any lines will get a letter advising about rights arising from a class action.

Basically, if you are ever invited to appear in a big-budget film or TV show you need to remember three things: do you have a line (otherwise it's low-paid day labor with free lunch and some celebrity-spotting fun), SAG minimum (get hired in accordance with the Screen Actor's Guild deal, or Actor's Equity on a TV program), Taft-Hartley (in case the producer says you need to already be a member of the actor's union). If you have a line and come under an industry agreement, you get a residual payment every time the film/tv show/commercial is rebroadcast. This can add up to a tidy sum if you are lucky enough to work on something that becomes a hit. If you get hired as an extra, you can try talking during the scene you're in to get a speaking credit, but you'd better add enough value to the scene that the director is willing to stick up for you when a producer objects. That's the basic difference between acting and just having a particular look. David Prowse can't act, sad to say; he's what people in the industry call a 'movable prop.'