At the moment there is a link on the Alignment Forum to apply for membership. You can apply by submitting papers, blog posts or comments. However, there is very little in the way of detail of what kind of work they expect they expect in order for you to have a reasonable chance of being invited. I'm not saying that there should be specific criteria as I'm in favour of the moderators using their judgement. However, more detail on what they are looking for would provide encouragement for people to work towards achieving the level of knowledge and good judgement that they expect for their members.