Winney, first of all, I think the alum idea is fantastic! Here's some tips.
--give it time! Usually NO organization is big and involved from the start. It just might take a while to get it going.
--Don't plan too many events. People lead very busy lives! Try an event once a month, or maybe even once every other month, and invite people far enough ahead of time that they can make time to attend.
--is everyone you invite well informed about the group and its purpose? Maybe some people are in the dark. Let them know your aim for the group.
--is your group only for alums of NPC/IFC groups? Or do you have a more broad focus? Because there is an SAI alum chapter there I know for sure. THere may also be alum chapters of other specialized groups. But, that's only if you want to include ALL Greeks. Obviously, if your aim is for alums of NPC sororities, then concentrate on them.
I hope this helps. Keep us updated on how things are going.