Skipping over what was already said, you might want to think about what goals, materials, activities, and expectations you'd want for your coven and then make a biography for it to post on a website, in the Advertisement forum, in your profile's biography, or any combination of the aforementioned.
When you tell people about your coven, you have to let them know what they'd be getting into and make it so that it properly conveys what you want while grabbing the attention of potential members. I'd avoid pushing people to join your coven, but if you talk to people, get to know them, and then invite them, you should be fine (in addition to putting it out in the open for people to come to you).
I'd advise really fleshing out all the details before publicly advertising for your coven so you can answer any questions people might have properly.
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