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’ (or the neighbors’ or the Canadians’), you are at the Smiths’ (or the neighbors’ or the Canadians’) place, and, as you suggest, the implied possession requires an apostrophe.