We are in the process of reviving a hope group that has had about 5 members for about 20 years or so, and we've been consistently seeing 10-20 women at events for about the past year. In my opinion, you would go about starting a young alumnae group in the same way that you would start a regular one, with the activities taylored to younger women, of course. So, I might be able to answer some questions for you. However, there is A LOT of helpful info on Anchors Online. Check out this page that explains the basics:
http://www.deltagamma.org/start_group.shtml
The page also directs you to some more in-depth info in the "anchored" (password protected) portion of the website.
Hope this helps!
Amanda