The rule for pluralizing a proper noun referring to an individual is the same as with common nouns. Simply add an "s" to the end. "There were many Michael Jacksons at the costume party." If the name ends in an "s", as with Santa Claus, add an "es" to the end. "There were many Santa Clauses in the mall this holiday season."
To pluralize a family name again add "s" or "es" to the end of the name just as with regular nouns. If the Smith family is having a party you can say "The Smiths are having a party." An exception occurs if the family name ends with "y". With normal nouns the "y" is changed to "ies". With proper family names, however, an "s" is added so that Kennedy becomes the Kennedys.
It is possible that singular places or things might need to be pluralized. An example of this would be an article where the author referred to both Southern and Northern California. Again the same rules apply as with family names, simply add an "s" or an "es". "Both Californias share a need for increased annual rainfall."
Some make the mistake of adding an apostrophe "s" to the end of a proper noun believing that this pluralizes it. This might come from seeing names of businesses with a proper noun family name like "Joe's" restaurant. The apostrophe "s" in these cases indicates possession, that Joe is the owner of the restaurant. To pluralizes Joe's restaurant you would follow the rule of of adding an "s": Joes.