A coven is a tight-knit, usually small (3-13 traditionally, occasionally up to 20) community of Witches, Wiccans or Vampires who gather regularly for religious and/or social occasions. Covens may gather for training purposes, or to celebrate Sabbats or Esbats and/or to work magic spells. They also gather to celebrate important rites of passage for their members.
Covens may be made up of unrelated Witches or one or several families. Children are inducted into the coven as they reach the appropriate age and so a coven may be passed down several generations. When covens are formed of unrelated members, new members may be brought in following an initiatory process. Most covens do not allow members under 18 unless they are related to someone already in the coven. Members are usually sworn to both secrecy and loyalty to varying degrees depending on the tradition.
When a coven gets too large or if there is a disagreement among the membership about any number of things, it may be split or “hive off” into two covens who may go their separate ways or unite periodically for major events.
List of Covens by State on Witchvox.com