The Jimmy Buffett-owned restaurant's web site implies that the buy-1-get-1 deal is only available for a limited time, although you can join the club anytime.
In addition, members receive a free welcome kit with "vouchers and other fun surprises," invitations to exclusive parties, free stuff on your birthday, chances to vote on new drink and menu items and email updates about specials and upcoming events.
Find the restaurant closest to you here.
I have to thank Frugal, Freebies and Deals for this one. Thanks FFD!