So far we know that Tesla, Aptera, and Illuminati Motor Works are all in, but there is reported to be over 60 teams from 9 countries participating. There are two categories, Mainstream vehicles and Alternative Vehicles, they each have their own set of rules, but think of the mainstream category as the vehicles that might actually be bought and sold in massive quantities in the future.
For the mainstream category the vehicles must have 4 wheels, hold 4 people, go from 0-60 in 12 seconds or less, have a top speed of 100 mph or higher, a range of at least 200 miles, there is also a cargo space restriction that says these vehicles must have at least 10 cubic feet of cargo space. On the alternative vehicle side it has to carry at least 2 people, hit a top speed of 80 MPH, have a range of 100 miles or more and at least 5 cubic feet of cargo space.
The main requirement regardless of category is that they must be able to go 100 mile or more on one gallon of gas or less. Aptera and Tesla already have the clear lead with Aptera already making vehicles with 3 wheels that hold 2 people and go 157 MPG using diesel and Tesla has their Roadster which is a 2-seater all electric vehicle that I am pretty sure already exceeds all of those standards, and they are rumored to have already been working on a 4-seater version.