No, you don't need to be a member but if you're not, you cannot participate in the regional point system. Plus, most contests will charge a non-imac member about and extra $5 or $10 bucks in contest fees.
My recommendation is if you plan to fly at a few contests, just join. It's well worth the the $20.
You can even register on line or send in via snail mail.
Lower left corner:
www.mini-iac.com