Not a bad idea Laci, and you can start where you work in the store, wear a big button that says "Online Book Club Forums.....intrigued?" and watch the naive public get reeled in! Or try walking around town wearing it and explain to anyone asking what it is. Hey presto new members, although there are at least 4 times the members around than this time last year so lets keep that up.
Note to everyone, if you do want to spread the word about the forum to friends or family, click the "Tell a Friend" link under the Site Navigation book on the upper left hand side of the main page, people may have missed that on the way to the forums.