Prince Bernhard was a member of the "Reiter-SS", a mounted unit of the SS and joined the Nazi party. He later also joined the National Socialist Motor Corps.  Various members of his family and friends were aligned with the Nazis prior to the Second World War, a number of whom attended the royal wedding. Protocol demanded that the prospective Prince-Consort be invited to an audience with his head of state, who at the time was Adolf Hitler. Hitler himself gives an account of the conversation he had with Bernhard in his Tischgesprache (Table Conversations). Table Conversations was a collection of monologues, remarks, and speeches Hitler gave during lunch or dinner to those invited to the table by him. Bernhard himself called Hitler a tyrant in a public speech on the BBC on 25 June 1940 after France fell to Germany.  The Prince's brother, Prince Aschwin of Lippe-Biesterfeld, was an officer in the German Army. Although the secret services on both sides were interested in this peculiar pair of brothers, no improper contacts or leaks of information were ever discovered. The Prince proved himself to be a loyal Dutch citizen and officer. He cut off relations with those members of his family who were enthusiastic Nazis. As a sign of his "Dutchness" he spoke only Dutch when negotiating the surrender of German forces in the Netherlands. The Prince was known to be very fond of smart uniforms and medals. He made a point of wearing his medals in the English court style, even though members of the Dutch armed forces wear their medals in the Prussian style.  The Prince's mother was no admirer of the Nazis and got into trouble for refusing to hoist the swastika flag on her country seat at Reckenwalde.

Answer this question "What did he do while in the "Reiter-SS?"" by extracting the answer from the text above.
He later also joined the National Socialist Motor Corps.