I was told they mail out membership packets about once a month. T-shirt aside, everything else you need is available online. if you're really concerned, bring the emailed receipt with you to the track. I had my temporary card at my first event.
or don't be a dummy like me and forget to print off your registration papers before the weekend. d'oh. if you have that, you can go straight to tech and avoid needing your ID or Membership card.