The proper form for NPC sororities would be "is". However, many women, Greek and non-Greek alike, so closely associate membership with being an active in college that they slip and say "was". At my alumnae panhellenic, some women say "I was a ZXY" - but we try to subtly correct them.
To further complicate things, it would be correct to say "She was a XYZ at the University of Hotness" because you would be referring to a specific period of time in the past - so I WAS a Gamma Phi Beta at Texas State University in the 80s. I AM a Gamma Phi who is now active with the Houston Alumnae Group and Katy Panhellenic. Katy Panhellenic has many members who WERE members at many different colleges and universities.