I'm a member of our Volunteer Services Team. You can participate in an alumnae chapter or club. There's nothing special you have to do, just find the closest one, contact someone from it to get information and go to a meeting
As an alumna member who is in school at a college with a chapter, you might consider advising or at least assisting the chapter with recruitment or philanthropy events! If you PM me with the school you'll be attending, I can find some contact information for you for both alumnae clubs/chapters in the area and advising opportunities!