When it comes to attention and what we do with it, the verbs focus and concentrate are very close in meaning. So close, in fact, they are often used interchangeably! Any difference in the terms is slight, but in some uses, concentrate may imply a more involved action. Think of the act of concentrating, which involves close mental application. To concentrate on a problem, then, is to bring all efforts and faculties to bear on it. Both concentrate and focus deal with the idea of bringing things toward a point of convergence; where they notably diverge is in focus's foray into optics (to focus the lens of a camera) and concentrate's dabbling in chemistry (to concentrate a sauce by boiling it down), which gives concentrate an intensifying edge.

To be angry is to experience strong feelings of displeasure. To be indignant is to feel deep anger at something considered unjust or offensive. Indignation is righteous anger, or anger that is morally justified. Someone who is indignant perceives insult or injury to their own or someone else's dignity. The term carries traces of scorn and contempt, and sometimes implies sanctimonious disdain. This range in meaning helps to explain why you might see indignant with a modifier, such as rightly indignant, justly indignant, and genuinely indignant. 

A group is a collection or assemblage of persons or things. When that number is on the larger side, the group may be called a bevy. Bevy's earliest use in English is as a collective noun for quail (behold, the bevy of quail!), similar to a covey of partridges. When in reference to people, bevy has historically described groups of women. While you will still encounter these specific uses, the word bevy has resisted being pigeonholed. The term is now widely used in a general sense to emphasize abundance in number, and can be found in reference to just about any type of assemblage or collection.