...we are a new (3 yrs old) and smaller Gamma Phi alum chapter located in the "southern crescent" area of atlanta (5 county area that actually form a crescent around atlanta's southside) -- there is a bigger alum chapter in metro Atlanta -- we are not near our two chapters in GA --one is in metro Atlanta, the other is an hour and a half north of us in Athens -- we still do things for them from afar: we do not wait to be asked -- we let them know we are here and want to help in any way we can, given the time and distance, without getting into their nearby alum chapter's knickers (but can too much really be done for collegiate chapters? I think not!) Anyways, we asked each chapter for a wish list of things they as a chapter wanted and needed but could not buy for whatever reason and we used our convention boutique profits (we made and sold Gamma Phi business card magnets and cleared about $600!) to buy both chapters nearly everything on their wish lists (we could not find some of the stuff they wanted). They both LOVED it! We also often send pick carnations (our flower) to the chapters for initiations; we periodically meet a few collegians on their turf for dinners and currently we are making and repairing the ritual robes and other ritual "stuff" used by both chapters. Since we are in the suburbs, we are more domesticated, so to speak, then our alum chapter in metro Atlanta.
As far ways to get more members to your events, I find that writing a personal note to each alum who is not involved helps -- I know we have a smaller pool to work with than you have there in Chicago, but perhaps you can split up the target alums and have several active members write the notes...in my notes I try to chat about something we might have in common, besides the sorority, and then I mention something the alum chapter is doing or has done and then I say how great it would be if you could join us! Schmoozing basically! It did/does work!
By the way, my inlaws live in Wheaton -- my F-I-L is one of the pastors at St John Lutheran there! Love That area!!