I can't think of a situation where someone would need to be told they are a passenger. If it's very obvious the people are passengers, and you tell them they are, they may take it the wrong way. Do you work on some transportation service (train, plane)?. You might say "Welcome passengers" or just "Welcome aboard".
Having said that, to apply to multiple people, "You are passengers" or "You all are passengers." . In Southern US dialect we say "Y'all are passengers." but that's only to be used in the South (interesting article about that here)
English curiously lacks a singular-plural distinction in the second person even though all other European languages have both: French ("tu" versus "vous"), German ("du" versus "ihr"), Russian ("ty" versus "vy"), and so on. Some time ago, English distinguished between "thou (art)" and "you (are)." "Thou" was second person singular and "you" was second person plural. Somewhere in the shuffle of history that crucial distinction was lost.