I took the SurveyGopher survey on SO, which offered an invite to Careers, but I forgot at the end to get it. Obviously I can't take the survey over again. Is there any way to contact an admin so that I can get the Careers invite?
You can request an invite at any time at http://careers.stackoverflow.com/cv/get-one. We review that list every few days, and if you're active on SO or any OSS project that's generally enough to warrant an invite.
In your case, I already went ahead and looked at your SO activity and sent you an invite.