Just to be clear, these aren't DCMM's events. It's an Autocrossers, Inc event that some people from DCMM will go to. Anyone can join it by following the link to their site and following the instructions there. There are a few other organizations that host autocross around DC, and the instructions are different for each. But basically, sign up, enter your car and it's about $35. Some ask for PayPal or credit card online, NCC requires membership which costs extra, and CDC allows you to just reserve and requires cash on site. All require you to work half of the event by replacing cones or the such. Most likely I will be there and can show you the ropes.
Also, anyone is invited to watch for free. They actually recommend just watching your first time to an event, and encourage talking to anyone since we all started from pretty much the same place and we're mostly more friendly than competitive.
http://autocrossersinc.com
http://www.wdcr-scca.org/events/autocross
http://nccautocross.com
http://capitaldrivingclub.com