"Joneses," as in "keeping up with the Joneses," refers to more than one Jones, just like a group of people with the last name of Smith would be the Smiths, except that with nouns that end in 's,' you ad 'es' to make it plural.
For possessive, when referring to one person with the last name of Jones, most sources prefer: Jones's. Some accept Jones'. As long as you're consistent in the usage, neither one will mark you as clueless, but a given publisher might change it in copy edits.
For possessive, when referring to several people with the last name of Jones, it would be the standard construction, just adding an apostrophe: Joneses'.
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