To become a priestess of an exsisting coven, you have to have been on the site and in the coven for a while showing that you can lead by example, showing and displaying that you are responsible, knowledgable, and good enough. It really helps by participating and helping others, knowing the answers or being able to find the, I.E. research is also very important.
But usually the position is offered to you, not by asking for it. In the case of the HP's being gone or have deserted the coven. You can always mail a mod and talk to them about the situation and see what they have to say.