I was a member of the SCCA committee that developed the Rallycross program in 2000. The very first SCCA sanctioned rallycrosses were held in the Denver area (SCCA National Headquarters were located here at the time and they wanted to keep a close eye on this new program). It went national in 2003 or so.
Rallycross was based on the solo (autocross) program but with the difference being it would be in the dirt. Our hope was that drivers would get hooked and move on to full performance rally. There are no special build standards for rally cross just some minimal safety standards like those found in autocross (sound car, stock seatbelt, Dot or Snell helmet, decent tires, etc.). The full goal was to allow normal street driven cars to participate. Naturally, car preparation is allowed, but not required, and may put you in another competition class just like in autocross. So, based on that, find out where you closest events are, show up and participate. Its really (pretty much) that simple.