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.
To explain is to make clear something that is not known or understood. A parent might explain to a disbelieving child why they cannot have ice cream for dinner (no nutritional value!). To expound is to give a methodical, detailed, scholarly explanation of something. It is a more elaborate action, with a slightly performative or presentational sense. A professor might expound theories, doctrines, or philosophies with lengthy verbosity. Legal experts might expound a law, and religious scholars might expound Scripture. But we also find it used humorously on occasion, in a slightly teasing or self-deprecating manner. While the use of expound with a preposition, as in expound on or expound upon, has some critics, pairings such as these have become increasingly common and are sure to be understood just fine.