They have an open application process at my school to be an active, voting member of the adcom. According to the Dean of admissions at our school, it looks good on your resume, but I don't see why residency programs would care. It's actually very time consuming to sit on the ADCOM, and it was very competetive my year, but a lot of students just volunteer to interview students and write a little feedback form. I wouldn't recommed the experience though, some of my classmates that are doing it say that it seems fun at first, but after a while all the applicants seem the same.