Further to what Ray has told you... from my experience... such as it is.. The contact for joining the Chetsociety and CAAS registration is Carol Pritcher (Mark's wife). At least it always seems to be Carol that answers the phone and emails. Their email is on the contact tab of the site Ray posted.
However, You need not worry about being accepted when you arrive for the convention. You can pay for both your membership and registration at the registration desk on the first morning of CAAS. The benefit of pre-registration is that you get a discount, but it is not very much (I think $20).. The big advantage of pre-registration is that it helps in the administration, because they can get your package ready ahead of time. Oh.. you also can get through the registration lineup quicker

If you are going to stay at the convention hotel (Sheraton Music City).. You should book your room now. Make sure you mention that you want the special CAAS attendee rate. I'm not sure how many rooms have been allocated for CAAS attendees, so that might be the most important thing to get out of the way early.
If you are planning on staying at a nearby hotel.. there are lots of them, so you need not worry about getting a place.