Field allowing multiple values but using autocomplete

My customer swears this used to work and that I have broken it.

There is a field that contains a long list of people who may have been present at a meeting. It is allowed unlimited values. They claim that it used to allow autocomplete to select people and had an "add another" button below it. Certainly, I see in some custom code 

$form['field_who_was_present'][LANGUAGE_NONE]['add_more']['#value'] = "Add Another Person";
which would kind of indicate that they might be right.

It is definitely not working now. Does anyone have any idea how to get it back to working as they want?

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