Most of the city is open camping.
Review this map: http://www.burningman.com/preparation/m ... index.html
All of the white areas are open camping, first-come, first-serve. Neighbors are expected to negotiate their space needs with each other and it is not uncommon for people there on the first day to hold space for an RV or two for people arriving mid-event.
However, the next task is for you to find your friends! They can post their location at Playa Info (near Center Camp Cafe). You or your GF can leave the parked RV a few blocks away (can't drive near the Cafe) and walk to Playa Info and look up the camp location your friends ended up at.
Failing that, mid-weekers generally have to camp along the outside of the city. eg 3:15 and J-street, or even L-street (they added an outer street last year).
This year's event is shaping up to sell out all of their tickets
. So it is going to be very full. Expect to be on the outermost ring or have your friends help you out.