Stratify means to divide into layers or classes. Announcing pay raises for only half of the employees in a company would be a quick way to stratify the office.
Stratify is often used in the context of geology, when materials in the earth’s crust stratify, or form layers. This phenomenon serves as an apt metaphor for other uses of the word, particularly as it's applied to social situations. If you attend a party for geologists and notice that the mineralogists are ignoring the petrologists (and the vulcanologist didn’t even bother to show up), you might conclude that something has caused this group of geologists to stratify.