You can enable or disable a book's Forum from the "Forum" page you can find here:
https://leanpub.com/YOUR_BOOK/forumA forum is only visible to people who have bought your book, and if the forum is enabled.
You don't have any special admin privileges in the forum itself - you're just like any other user who can create new topics and make contribute comments!
You can also find a link to the Forum page under "Community" on your book's Overview page:
