If you're now a paid up member, you can find your membership number by clicking Settings at the top right of the page, scroll down the menu on the left until you see Paid Subscriptions, then it will tell you in there. Signing up is as Sam mentioned above, although if you can't find the sign up form, you can always just email the EO (Jonno, Alan or James/Grant) to either state that you're coming or ask where you can find the form.