Not to be a broken record, but try to see if they will reconsider. Not joining isn't going to hurt you in the long run as there are tons of people who have succeeded without being in it. However, it is one of those things that looks good on a resume (or in the case of law firms included in your bio on the website), even if thats all you do with it. Personally I joined the history and econ honor societies (I wasn't smart enough for phi beta kappa) basically b/c they will look good on a resume. Whether it has helped in getting my current job or in getting future jobs I don't know, but it cant hurt.