Both Mercedes Benz and Auto Union conducted driver tests at the Nürburgring in October 1936. Auto Union took just two days to select Müller as their new driver for 1937, from five candidates.
Mercedes Benz stayed for a fortnight and invited a total of thirty drivers, of whom ten were allowed to drive the GP cars. Seaman and Kautz were selected as the best two, and joined the team. Unfortunately, I don't have a full list: Leif Snellman might -
The only name I can add to those on Leif's page is TP Cholmondeley-Tapper, who was invited by Mercedes Benz, but declined and went skiing instead (!!!!)