[aopinthesky]

Zeta,
I agree with Christin, you have received some EXCELLENT advice and ideas based specifically on the question that you asked. Christin, in particular, gave you some outstanding pointers for operating an alum chapter where the members are all in the same area as well as what you can do if they are spread around the country. I am assuming that your sorority is a local without other chapters? You have to keep in mind that members of IFC and NPC groups go through their respective Inter/national Headquarters to set up alum groups. Unless someone like Christin (who really knows her stuff) reads your post, the advice you are likely to get is to call your Headquarters!
You might also post this in the Locals area in case someone missed it here. Another local group might have some ideas geared more toward what it is you are looking for. I agree that talking to your brother fraternity would be a great idea, since they function so well.