Short answers, pulled from the story.
The Anti-Comintern Pact was signed on the 25th of November 1936 by Joachim von Ribbentrop for Germany and Kintomo Mushanokoji for Japan. It was signed in the offices of the Dienststelle Ribbentrop rather than the German foreign ministry.
The secret additional protocol specifically named the Soviet Union as the target and established a defensive arrangement: if either Germany or Japan was subjected to an unprovoked attack by the USSR, the other party would take no measures to ease the Soviet position. It also prohibited either signatory from concluding political treaties with the Soviet Union without the other party's consent. No other country that later joined the pact ever signed this secret protocol.
Hiroshi Oshima was Japan's military attache in Berlin and the single most important individual on the Japanese side of the negotiations. Acting without authorization from Japan's ambassador, he used back-channel contacts through Friedrich Wilhelm Hack to build relationships with Ribbentrop, Wilhelm Canaris, and Werner von Blomberg. He later became Japan's ambassador to Germany in 1938-1939 and again in 1941-1945.
Germany and Japan were the original signatories in November 1936. Italy joined on the 6th of November 1937 and was legally recognized as an original signatory under its entry terms. Spain and Hungary joined in 1939. Several additional countries joined during the Second World War, with the pact formally renewed in November 1941.
The Molotov-Ribbentrop Pact of August 1939 caused Japan to distance itself sharply from Germany, because Germany had concluded a non-aggression agreement with the Soviet Union, the very country the Anti-Comintern Pact was designed to contain. The September 1940 Tripartite Pact subsequently redirected the alliance's focus toward the United States as the primary threat.
Soviet Foreign Minister Maxim Litvinov publicly declared the day after the signing that the published text was "only a camouflage for another agreement which was simultaneously discussed." Internally, the USSR viewed the pact as an attempted encirclement by Germany and Japan. Soviet-Japanese trade declined sharply in the years following the pact, with Japanese imports from European Russia by 1939 falling to their lowest level since 1914.