Wait for the next opening (While you do that, you can post a link to your app in the group comments for critiques!)
When the group is open, send in a join request. In the request comments, fill out this form: Name: Link to App: How you found the group:
We will look over your request and your app, if everything is in order, we will accept your request. If not, we'll tell you what's wrong and when you fix it, we can accept you. Right now, prospective members are allowed to resubmit their join requests as many times as they wish during the duration of an opening.