Learn Spanish (Latin America): Nadie entra sin el pase, menos los miembros. - Nobody enters without the pass, except the members.

You are right that menos usually means "less" or "minus" (like in math). However, in everyday Spanish, it is also very commonly used to mean "except" or "but" when you are excluding someone or something from a larger group. You can think of it literally as "minus the members."

Could I use excepto or salvo instead of menos in this sentence?

Yes, absolutely! Excepto, salvo, and menos are completely interchangeable when used to mean "except." Menos is slightly more casual and very common in everyday spoken Latin American Spanish, but saying ...excepto los miembros or ...salvo los miembros is also perfectly correct and means the exact same thing.

Why do we need the article los in menos los miembros? In English, we could just say "except members."

In Spanish, you generally need to use the definite article (el, la, los, las) when talking about a specific group or a noun in a general sense. When using exclusion words like menos, you don't drop the article just because it's a prepositional phrase. You must include los to define who the members are.

Since Spanish uses double negatives, why isn't it "nadie no entra"?