You can add forums to your guild site on the Admin menu:

• Click Features.
• Click Forums from the column on the left.
  
• Click Forums again from the menu that appears (this is the actual link that will bring you to the Forums tab).
• Click Create Forum under the category where you would like to add the new forum.

From here, you need to enter a few details:

• The forum name
• A short description of the forum (like "Post raid strategies here!" or "For anything not relating to badgers.")

You can also establish whether or not it's a subforum to a current forum, change the category, etc.

Once you've done this, click the Create new forum button and you're done!