A few of the NPC sororities do allow graduate students to pledge, but this does not guarantee a bid to membership. I would advise against rushing and instead focus on your wedding, graduate program and meaningful involvment in professional societies that will enable you to get the connections you need to get a job once you finish your program.
However, there are still opportunities for you to get involved with becoming a sorority member with a
non-collegiate sorority. You will enjoy the benefits of social enrichment, service, sisterhood and leadership through membership in these sororities, just as you would in a collegiate-based group.
Good luck! You might also consider a community service group like the Junior League or joining the junior board of a charitable organization in your community like the local chapter of the American Cancer Society, the Ronald McDonald House, etc.