Looking to join a collegiate sorority as an undergrad. AND grad. student.

[adpiucf]

I wouldn't recommend that you pursue NPC sorority membership.

Grad students are not eligible for membership at 95% of the NPC sororities. The ones who may take graduate members typically don't.

You may want to consider a community activity like a non-collegiate activity. Also, look at professional organizations and clubs within your graduate program. You will be in a close-knit group of people for the next few years, and you will have considerably larger responsibilities to your classes than in undergrad.