Yes - any BMW CCA member can attend any driving school hosted by any chapter. In some cases the hosting chapter will require you to be a member of their chapter additionally, and the fee to be registered in more than one chapter is small (~$15).
Windy City BMW CCA does not post the applications for their events on their website. If you contact them they will fax or mail you the application when it comes out if you are not a Windy City BMW CCA chapter member. Their newsletter contains the application, so chapter members (who get the newsletter) automatically get the app. The Grattan application is supposed to be out in the next two weeks or so...