Are these singing groups a class you have to register for (which is what ACES makes it seem like) or an extracurricular you audition for early in the semester and which you don’t sign up for as a class (which is what the chorale website makes it seem like)?
anyone? thanks!
bump /10char
Most of them are extra-curricular activities that you have to audition for. Some of the very top a capella groups are very very competitive. I don’t think chorale is nearly as competitive. It’s possible that there are some courses that also give you exposure to those areas as well, but I’m not aware of them. Here’s a summary of a few of the groups (as of 2013):
http://www.duarts.org/category/a-cappella-council/
I recall The Pitchforks, for instance, being very popular and tough to get into (all male group). They are considered one of the top male collegiate a capella groups in the country. Here’s a sample from 2012, very professional sounding that’s also on Pandora:
https://www.youtube.com/watch?t=13&v=48wVEqjG0Rs
A capella at schools like Duke is very “in” – the movie Pitch Perfect gives an accurate representation. 
(not exactly, but it does take some of the characteristics and blows them up with great humor). I would recommend visiting the websites for more information and contacting the heads, but they should also have a presence on campus during orientation week/activities fair that you can sign up for auditions, so it’s still quite early now.
Great summary, @bluedog.
@RunningForLife: You may also wish to consider some of the singing groups affiliated with various Duke student religious organizations (including the Chapel Choir).