If you do not want everyone registering for your site to have Writer Privileges, then in
Dashboard > WriteShare > Settings
leave UNCHECKED: New users are authors by default
Here is how a subscriber can manually apply for Writer Privileges and be approved by a Site Admin.
Once registered for your site, the subscriber should to to their profile pages at http://yoursite.com/profile/ to find a link to request Writer Privileges:
After Requesting the subscriber will see
The Site Admin will receive notification:
The link will take the Site Admin to the USER PROFILE. Note, the very bottom of the User options is a field to allow Writer Privileges. Be sure to check the box and UPDATE user.
The subscriber, now writer, will receive email notification of approval:
The subscriber is now a writer and can access your http://yoursite.com/write/ page.
Here are the instructions on Posting and Editing.