I don't know if there is a special bus from the circuit to the town but there is a regular bus. If you walk the road passing the main entrance northwards (is it D139?) you'll find bus #6 and it seems to go pretty often. It's a ten minute walk from the main entrance to the bus stop.
For travelling back to the circuit at friday night,
there are a lot of locals who try to earn a bit of extra money by being a taxidriver for the night.
Last year we went with 5 people in one car plus driver for about half the price of a real taxi