Autocross/Solo II events are low to medium speed auto racing events; they are often run on parking lots and airport runways, although street events and events at Go Kart tracks sometimes take place. Generally a course will be defined using traffic cones. One driver at a time negotiates a course laid out with the cones, or pylons, testing their skill against the clock. Time penalties are charged for disturbing cones. In most regions, the penalty is 2 seconds per cone, although in some places it may be 1 second. There is an upper speed guideline for Solo II which is intended to keep speeds in a domain that most drivers might have encountered on the streets and highways; the fastest cars at a Solo II should not get much over 70mph. Generally, each driver takes three or four runs at an event. A driver is awarded the best time of all runs taken.
Most autocrossers are driving/racing enthusiasts who enjoy motorsports competition. In fact, a large percentage of participants do not even own a "racecar"! They race the car they drive on the street. Some autocrossers are serious road race drivers who want to practice technique. Most are like you and me: laypeople with an itch for speed!
Autocross is an inexpensive, safe way to experience racing. It helps you discover your car's capabilities and limitations, making you a better, safer driver on the road. And it develops your own driving ability. Many would-be race drivers use it as a jump point into the sport of road racing. But probably the greatest thrill of autocross is the challenge of beating your own time. It's fun!
Costs vary widely around the country. Typical cost for a single day event is probably ten or twenty dollars. The sponsoring club has expenses like site rental and insurance. Costs for insurance are not insignificant in lawsuit-happy America. Keeping safe courses and speeds is important to the survival of the sport.
It costs almost nothing to start autocrossing, and since other cars are not on the course with you, there is almost no risk of damage to your car. To get started, just go to your nearest event. Ask questions, and if you brought your car with you, you might even be able to register and race right then and there! To find out where your nearest event is, subscribe to the mailing list, and post a message with the subject line "Hometown, State Events?" You should get at least one lead.
Autocrosses do not typically charge for spectators. Just go. You may be asked to sign an insurance waiver.
Membership in the organizing club is required in some places, but not necessary at most of the local events in the country. Check with the sponsoring club. However, membership has its benefits; event fee discounts, newsletter subscriptions, etc.
For autocross/Solo II events, no SCCA licence is necessary.
At the regional level, Solo II is very much a grassroots sport, but a few large SCCA Regions (San Francisco for one) require SCCA membership to participate. Check with your sponsoring club. At the Divisional and National level, drivers must be SCCA members to compete.
From the Autocross FAQ