Hola,
I’m a native Spanish speaker from Spain (I live in the U.S., spoke English all my life with a native English speaking father and my English could be better than my Spanish). Since I am Spanish, we use vosotros. While I heard people in the U.S. learn “ustedes comen”, I would say “vosotros coméis”.
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Nah, ambas terminas son correctas pero la RAE recomienda usar “español”.
“Castellano” is the word used for Spanish in general in many countries in Latin America though