When Pet, decides he wants to go through the mess of sorting out and deleberating all the non sense that took place last time covens were open to be made, either that or it has to do with a file space or transfer allowence issue, I have heard both, even other stories as why the option was closed in the first place.
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.