If you click on any of the tabs at the top of the page (Home, Blog, FJ Ambassadors, etc), you'll be redirected to an entry page for that particular section. On the righthand side of these pages, you'll see a large button (under your avatar) that says "Spread the Word, Invite Your Friends." Just click on this button, enter your friend's email address and a brief message, hit Submit, and you're all set!
Hope this helps! The more the merrier in the FJ Community.
Thanks for helping to spread the word.