In the book Frindle, author Andrew Clements shows us that society decides what words get added to the dictionary.
The question of how dictionaries are created begins being explored in chapter 3 when Nick tries to distract his fifth-grade English teacher from assigning homework by asking what he thinks is a "perfect thought-grenade":
.. where did all those words [in the dictionaries] come from? (p. 17)
Nick is then assigned to explore the answer to his own question by writing and presenting a report on the dictionary. Nick's research shows him that scholars, like Samuel Johnson in the 1700s, compiled dictionaries, but Johnson's dictionary was compiled from many other English dictionaries before him. When Nick asks his next "thought-grenade" concerning who decides what words mean, Mrs. Granger explains the role ordinary people and the rest of society play in deciding what words mean and what words get added to the dictionary. Mrs. Granger explains the role society plays in her following answer to Nick's question:
Who says dog means dog? You do, Nicholas. You and I and everyone in this class and this school and in this town and this state and this country. We all agree. (p. 27-28)
In other words, as soon as all of society agrees on the meaning of a specific word and begins using that word to mean that thing frequently enough, the word gets added to the dictionary, which has been compiled for centuries by scholars.
Nick demonstrates society's role in adding words to the dictionary when he invents a new word to mean pen, frindle, and convinces all of society around him to use the word instead of the word pen. By the time he is 21 years old, Nick discovers that his made-up word has been added to the dictionary.