You can enable or disable a book's Forum from the "Forum" page you can find here:
https://leanpub.com/YOUR_BOOK/forumYou can also find a link to the Forum page under "Community" on your book's Overview page:
On that page, you will see something like this:
To enable your book's Forum, tick the "Forums enabled" box and click the blue "Submit" button. Afterwards, you will see that you have the option to disable your Forum any time, by un-ticking the "Forums enabled" box and then clicking the "Submit" button again:
Once you've enabled your Forum, anyone who buys your book will be able to join it by selecting the book in their Leanpub library https://leanpub.com/user_dashboard/library and then clicking the "Go to the Discussion Forum" link:
Forum links are stable and look like this:
https://community.leanpub.com/c/manual
In the Forum, you and your readers can post topics and discuss them. Here's an example of a new Forum:
An enabled Forum is only visible to people who have bought your book. Access to the Forum is limited to your readers only. Anyone else who follows the Forum link will see something like this:
Please note that 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.