Short answers, pulled from the story.
Joachim von Ribbentrop signed for Germany, Galeazzo Ciano signed for Italy, and Saburō Kurusu signed for Japan. Adolf Hitler observed the signing ceremony in Berlin on the 27th of September 1940.
Hungary joined on the 20th of November 1940, followed by Romania on the 23rd of November 1940. Slovakia entered the alliance on the 24th of November 1940, and Bulgaria signed in Vienna on the 1st of March 1941.
A military coup d'état overthrew the regime just two days after Dragiša Cvetković signed the pact in Vienna on the 25th of March 1941. The new government under General Dušan Simović refused to ratify the agreement and began negotiations with the United Kingdom and the Soviet Union.
The Finnish government refused multiple German requests to sign the Tripartite Pact because they viewed their war as separate from World War II. They maintained diplomatic ties with the United States until June 1944 while joining Operation Barbarossa on the 25th of June 1941.
The defensive clauses were never invoked during the entire duration of World War II. Historian Paul W. Schroeder described the pact as rapidly declining from importance in late 1940 to nominal existence by late 1941.