Yes. Northbrook is a park district facility, so whenever there isn't an organized event, it is open to the public. Visit northbrookvelodrome.org for all the details.
As far as racing, Northbrook gets an excellent turnout, usually with 20-25 riders in the P/1/2 fields, more in the 3s and 4s. And our women's turnouts have been extraordinary this season, with 12-15 riders in the Cat. 4 women's fields. We're very proud of our parking lot.
Cat. 5 riders have to complete a certification program before upgrading to Cat. 4 and being allowed to race on Thursday nights. This is (I believe) 5 race nights and 5 training sessions (we run two, one on Monday for everyone and one on Tuesday for elite riders, and lower categories with prior approval).
Overall, the racing is excellent (and fast as hell), and it's a friendly joint.
There is also Kenosha, that hosts racing on Tuesdays. It's a traditional 333 with a (again, don't hold me to this) 28-degree banking (vs Northbrook's 400m/382 in the pole lane). We do motor work there on Saturdays. The races on Tuesday are very low-key and friendly, even more than Northbrook. It can be quite a haul in rush-hour, though.
Hope that helps.