"...They are recently retired and very bored. I'm a wardrobe assistant who's just breaking in myself, so I don't know how to help them, but they think it would be fun."
I don't know what they retired from, so I'll assume it's not entertainment. Because of that assumption, I'm guessing they have no relationships in the industry, and probably no specific talent like Cinematography or Editing. Starting from scratch, may be a bit of a stretch in a competitive, unionized, industry. YET, they can still be in the movies.
Yes, it's true. If they are willing to listen, take direction, and "pretend" to talk, they can work as background artists. It sounds like you live in an entertainment city, being a wardrobe assistant. If your grandparents do too, they can register with an extras casting agency. Central Casting is one of the biggies in LA. Google "Extras Casting Agency."
What makes it even easier for them, is they don't need to be in a union when they start out. It's certainly something to aspire to, but not a necessity for breaking in.
For more tips and articles by top entertainment industry coach, The Greenlight Coach, visit www.thegreenightcoachblog.com