The Grand Coven refers to an established community of witches that are structured according to their members' affinity for magic. This particular coven is the supreme command of all witchcraft, and without their sanction, witches can be forbidden to practice.
According to Rowena MacLeod, the Grand Coven classifies its members into three recognized kinds of witches—the borrowers, the naturals and the students.
Witches that belong to this category harness demonic powers to practice witchcraft.
According to Rowena, these witches are born with a natural gift for magic. Rowena said these were very rare and claimed she was considered one of them.
These are witches that had no natural ability but, with practice, training and with a mentor approved by the coven, could produce a modicum of "witchly power".
Currently very little is known about them. Since it is known that a coven is a group of witches, and this particular one is known as The Grand Coven. It is known that they condemned Rowena for her methods and threw her out of the coven, forbidding her from doing magic and forming her own coven. They later sent Katja to deal with Rowena when she was stirring up trouble again.
It's revealed that they are considered the supreme command of all witches, but were severely weakened due to the efforts of the Men of Letters who used their political power to rile the church into attacking. Most of the Coven's best were burned at the stake or forced into hiding while their most powerful spells and potions were plundered and scattered into bunkers scattered across the world. A couple of Grand Coven witches tell Rowena, at one point, that they were "being burned or hung by the thousands."
However, the coven's surviving members were led by Olivette and attempted to ransack Men of Letters's crypts and bunkers which were scattered around the world to reclaim their former possessions, even after the Men of Letters' downfall in 1958. However, they were repelled by Men of Letters' warding spells. After Olivette was cursed by Rowena, the coven was left without a leader, and the future of the coven became uncertain, even if the coven still had some superior members.
Rowena, with her newfound power and spells from the Book of the Damned, attempted to recruit active Grand Coven members in an attempt to create the Mega Coven, led by herself. She was eventually killed by Lucifer after helping him escape his cage. However, by a seal, Rowena was revived. While attempting to recruit some Grand Coven witches, Rowena mentions that the Grand Coven is "utterly fallen apart" since Olivette's disappearance.
In 2018, Rowena sought the Black Grimoire in order to gain access to a ritual within that would remove the magical binding that the Grand Coven had placed upon her powers. Though he refused to give Rowena the Black Grimoire, the hunter, Sam Winchester, provided Rowena with the page containing the ritual. Rowena was able to use it to remove the Grand Coven's binding and regain her full powers.
Witchly Hierarchy 
It is strongly implied there is a hierarchy within the Grand Coven. Olivette is revealed to be a higher-ranking member and the High Priestess. On the other hand, Claudette and Magda described themselves as grunts. Katja proved to be subservient to leadership of the Coven, travelling to US to locate and deal with Rowena, by their ask. It is confirmed the existence of the positions as High Priestess, officer and sergeant-at-arms within the Coven.
- Rowena MacLeod (expelled; deceased)
- Letitia di Albioni (mentioned only)
- Katja (deceased)
- Olivette (transmogrified into a hamster)
- Nadia (mentioned, deceased)
- Clea (deceased)
- Clea's Friends (deceased)
- Charisse (deceased)
- Melodie (deceased)
- Thomasin (deceased)
- Magda (deceased)
- Three Unnamed Witches (deceased)
- Season 10
- Season 11
- Season 13
- Various & Sundry Villains (mentioned only)
- According to Olivette, witch-on-witch murder is an anathema to the Grand Coven.
- It is unknown if male witches are accepted within the Grand Coven, as so far only female members were shown.