Generally, if a post is made that has some meaningful content to it, it is considered as working to the 30 post requirement. I think that the phrase used was "meaningful content" in one of the posts that was made regarding how to get access to the stories section.
In my opinion, writing a story, even if it is just short parts, is a good way to get to the 30 posts, and it shows that the person is willing to make an effort to contribute to the whole group. Other ways are to find pictures or articles about braces, or things related to braces that are of interest. Everybody has to find their own way to contribute.
I enjoy writing, and not everything that I write can be posted here. All of my stories are in my collection, as well as many stories from other writers, some who have sent me the original files.