Possessives and Attributives

Q. The information posted on the Possessives and Attributives web page comes close to answering my question, but I would appreciate a more detailed explanation: Did we have dinner at the Smiths or at the Smiths’? I am tempted to omit the apostrophe if I consider the preposition “at” equivalent to German bei + dative plural, French chez, Italian da, etc. But if “at the Smiths’” is shorthand for “at the Smiths’ house,” perhaps I need an apostrophe. Is “Smiths” functioning as a genitive or an attributive adjective? What if, instead of “Smiths,” I refer to a group of people (residents, occupants) by some other word, e.g., We had dinner at the neighbors, Canadians, etc?

A. Throwing a dinner “at the Smiths” works if you’re describing a food fight, but if you are at the Smiths’, you are at the Smiths’ place, and, as you suggest, the implied possession requires an apostrophe.