But what if their names are Marilyn Monroes or Arthur Millars? Is the plural then "Marilyn Munrosess" and "Arthur Millarss"? Or alternatively, "Marilyn Munroeses" or "Arthur Millarses"? In which case you might drop the "e" and they become "Marilyn Munroes's" and "Arthur Millars's" and the apostrophre represents the missing "e" in the same way as contractions and "don't" is "does not"...
Just askin' ...
My two cents:
I'm going over to Arthur Miller's house. (singular possessive)
Have you met the Millers? (plural)
I'm going over to the Millers's house. (plural possessive)
I'm going over to Henry James's house. (singular possessive, name ends in s)
Have you met the Jameses? (plural, name ends in s)
I'm going over to the Jameses' house. (plural possessive, name ends in s)
Hope that helps.