The $40 membership is required to run at NASA events. NASA has a few member benefits, one is a years subscription to Grassroots Motorsports Magazine ($19.95). This is were they publish their news letter. You can see more details on what you get from the membership here:
http://www.nasaproracing.com/aboutna..._benefits.html
And get your membership here:
https://www.nasaproracing.com/joinrenew.html
As far as event registration, you do that here, under the Midwest section:
http://www.nasaproracing.com/nasa_event/list_events
Other areas have their registration open already, and the page says registration for Midwest events will be open soon. The NASA midwest page will also have the regitration info on it, but when you go to register it will take to you the same place the above link will.