Also (and if I'm out of my lane here I will delete this) I know that Sigma Kappa is expanding to Arizona State this spring (so right now), have you gone to any of their rush events? Sometimes colonies do take senior members in order to get a normal class distribution. It is by no means a sure thing (I know that Sigma while colonizing at Drexel University in Philadelphia only took about 6 seniors, and this was fall semester) but it might be worth a try if bid day did not happen for them yet.
Otherwise there are some sororities that allow for graduate membership but mine is not one of them so I cannot give you specific details from personal experience. I also believe (and correct me if I'm wrong) that the chapter still may not choose to take graduate members, even if their organization allows them to.
Furthermore graduate level classes may not give you adequate time to pursue active sorority membership, even if you did have the desire. Also as the population of a collegiate chapter will (likely) be mostly undergraduates, you may not feel as much of a connection with them emotionally and socially. Just some things to think about as you're considering this. Best of luck to you, whatever you decide and let us know how it goes!
