You are correct that often the only way to join such groups is to email the Leader and ask to be invited.
A day will come when a category will die, and likely be archived in the same way that the category "EarthCaches" was archived. I would rather see an opportunity to 're-open' enrollment and allow under serviced groups be replaced, in the same way that "Law and Order" seems to keep going and going, while replacing the entire cast several times.
Until then, all you can do is beg the Leader (or some officers can do it too) to invite you to join.

BQ