In my opinion, and based on experience I've witnessed from being in a successful online coven years ago (I left because I just didn't feel in sync with the coven), the only way to have and keep a strong, healthy VERY ACTIVE in participation (not just "checking-in") the leaders of the coven must have like-minded genuine friends to join as a base foundation.
I'm talking friends who chatter with you daily on, say, Twitter, or some other social network. Get these kind of friends in and the coven will be already active with very eager participants. Priestesses won't have to worry about trying to come up with incentives to make people participate or threatening to delete them from the coven if they don't participate. Participatating would be a natural given in the way many here can't wait to post on Twitter, Facebook, etc. Coven members NEED to have a deep passion to participate and all the incentives and threats in the world won't get people to participate much without hardcore real friendship as a base. What WILL get people to participate is having the love of friends to chat spellwork and witchy talk to in the coven. You need to make it so people can't wait to log in and chat to other fellow coven members, but it starts with people you already know and love personally.
Then people you don't know who are the type who sincerely want to not only join, but want to participate as well will eventually want to join in on the high activity of these friends in the coven.
So my advice for those who are applying to be priestesses for these inactive covens, make sure you get your chatty friends to join to, as that will be the solid thread that will keep your coven healthy and very active!
The coven I used to be in are all women, mostly over 30 but they do have a few teens. The priestess' best friend is in the coven, and she hangs out offline with other members of the coven too. Many live in the same state as others so whenever coven members travel, they visit other members. They mostly discuss witchcraft, spellwork, etc. on the online boards but also talk about very personal things going on in their lives (work, relationships, family life, etc. LOTS of personal advice is given as well which increases the bond)...Only members can read their posts, so all talk remains private within the coven members. So because the core members are so close, that coven is one of the rare few on the Internet that has remained active for many years, and although members do come and go, it never has threatened their livelihood as a coven because of the strong bond of real friendship being the base for the coven.
