En masse is an adverb used to describe a group doing an action all at once or as a single entity. It is an appositive, which means it comes after the word it modifies. People run en masse. It comes from French and literally means in a mass.
Sometimes en masse is used as an adjective. The dictionary does not define this as an officially accepted use, but it is common enough that the meaning should be clear to readers. Convention seems to dictate that when en masse is an adjective it loses the appositive quality and comes before the noun it modifies.