We meet this most often now in the set phrase kith and kin. What that means isn’t always obvious. Some use it as no more than a wordy way of describing one’s relatives; for others, it refers also to a wider group of friends and acquaintances. It can also have the sense of a group of people with the same ethnic origin, usually one under a threat of some kind.
As a phrase, kith and kin has been in the language for more than 600 years, the first known user being William Langland in his poem Piers Plowman of 1362. Kith is Old English, cýðð, which meant knowledge or information. It’s closely related to couth, which meant something or somebody known to the speaker. Its opposite uncouth then meant an unknown or unfamiliar person or place but in the fourteenth century came to mean something distasteful and shortly afterwards an odd, awkward, or clumsy individual; our modern sense of someone ill-mannered or lacking in refinement and grace came along in the eighteenth century.
Kith has gone through several stages. Starting with knowledge, it took on the idea of country that’s known or familiar, one’s native land or home. A small further step shifted it from the land to its people, one’s countrymen and women, and one more shift limited it to the group a person knows or knows of, his or her friends, neighbours and acquaintances.
This last sense is still in use, which makes kith and kin a wider group than just kinfolk or relatives but includes a penumbral group shading from close friends to distant acquaintances.