To marshal at the Circuit of the Americas in Austin, Texas you have a few options. Either work or volunteer.
First, if you are a member of the SCCA you are welcome to volunteer any club events that are held at the track. Similarly, you are welcome to volunteer major events like F1, WEC or MotoGP.
Otherwise, you should consider getting a job directly with the circuit. COTA provides it’s own marshals for track rentals and some of the other events held at the track. The pay is minimal.