No on the membership requirement.
Most clubs will let you race, unless it's a larger Divisional or Regional type event.
Membership does have it's priveleges.
It's generally less expensive (charge different rates for non-members),
to the point that in a half-dozen events, you'll make up for the cost of the membership.
You also get an annual subscription to Sports Car Mag, and...STICKERS!
Seriously though, if you get to an event, and like it (you will), I'd highly recommend joining up.