A coven is a gathering or group of witches who get together and practice witchcraft and magic. A complete coven of witches is four members, one to represent each cardinal point: north, south, east and west. This is regarded as making a complete witch's circle, which is required for performing spells effectively. The more witches a coven has, the more powerful it will be, though it's implied that a coven must have at least four members. Prior to the arrival of Sarah Bailey, Bonnie, Nancy Downs and Rochelle's attempts to perform spells were unsuccessful. As Sarah is also a natural witch, it is implied that the presence of a witch born with their abilities can enhance the power of the others, though it is not strictly necessary for a natural witch to be a part of a coven for it to be successful.
Forming a Coven[]

The initiation ritual
If a group of witches wish to make a coven, they must perform an initiation ritual, which all potential members must take part in. This involves each aspiring member taking a ceremonial knife called an athame, pointing it at another member's heart and stating "It is better that you should rush upon this blade than enter the circle with fear in you heart. How do you enter?" The correct response is "With perfect love and perfect trust." The two members then kiss each other upon the cheek and the process is repeated until all members have been initiated. The four girls in the film are shown performing the initiation in a wood, presumably to be closer to nature, though it is unknown if this a compulsory requirement.
Coven Rules[]
Members of a coven are supposed to trust and respect one another. They are regarded as being as close as family; female coven members refer to one another as "sisters" - it can be assumed that male members would refer to each other as "brothers". Presumably, individual covens will make up their own specific rules and laws, though these are likely based upon the laws of magic, such as the Rule of Threefold. Nancy Downs claims that "in the old days", if a witch betrayed her coven, such as by trying to perform potentially harmful spells against the others, the other members would kill her/him. However, it is unclear how true this is, especially seeing how dysfunctional the girls' coven had become at this point.