A cadre is a group of specially trained people prepared to lead or instruct others. Examples could be a military unit, a group of scientists, or a band of insurgents.
Just looking at the "re" at the end of cadre suggests the word has some French roots, and so it does. The word, stemming from the Latin quadrum, meaning "a square," also meant "framework." This idea of a frame or border came to suggest the word as meaning a small group that creates the framework for a larger one, much like a picture frame. Your cadre of rebels consists of two cats and a stuffed bear . . . at least they'll be good at keeping secrets.