Names are nouns, and they are made plural and possessive like other regular nouns. For instance, four men named John are four Johns, and the hats the Johns are wearing are the four Johns’ hats. This is simple enough, yet when it comes to last names, there are several common errors that many people make.
Plural last names
Making a last name plural should never involve an apostrophe. The members of the Johnson and Smith families, for instance, are the Johnsons and the Smiths, not the Johnson’s and the Smith’s.
Last names ending in s are no different. Most nouns ending in s are pluralized by adding es. This applies to last names as well. The members of the Edwards and Doss families are the Edwardses and the Dosses, not the Edwards’s and the Doss’s. Your spell check might disapprove of the correct forms, but spell check is wrong on this matter. If you understandably find words like Edwardses a little too awkward, consider rewording to avoid the plural. For instance, the Edwardses can become the Edwards family or the Edwards household.
There is one important way in which plural last names differ from other plurals: the last syllable of names ending in y does not become ies when made plural. The members of the Kennedy and the Clancy families are the Kennedys and the Clancys, not the Kennedies and the Clancies.
Possessive last names
When it comes to possessives, last names are, again, like other nouns. We make names that don’t end in s possessive by adding ‘s—for example, Mr. Johnson’s hat, Ms. Smith’s umbrella. There are two schools of thought regarding singular nouns, including singular last names, ending in s. Some make these words possessive by adding ‘s (Mr. Jones’s house, Ms. Doss’s car), and some add only an apostrophe (Mr. Jones’ house, Ms. Doss’ car). Some English reference books recommend the former approach and some the latter, and some say both are acceptable. Purists tend to prefer the former, which is more traditional.