Short answers, pulled from the story.
By 1965, the PKI had approximately three million members and, together with its affiliated mass organizations, claimed a total membership representing one-fifth of the Indonesian population. The United States Department of State estimated party membership at about two million, or 3.8 percent of Indonesia's working-age population.
The PKI grew out of the Indies Social Democratic Association, founded in 1914 by Dutch socialist Henk Sneevliet. The organization formally became the Partai Komunis Indonesia in 1924, after changing its name at the Fifth Comintern Congress.
General Suharto formally banned the PKI on the 12th of March 1966 following the 30th of September Movement coup attempt. An estimated 500,000 people were killed in the anti-communist purge that followed, and a 2016 tribunal in The Hague concluded the massacres were crimes against humanity.
The U.S. embassy in Jakarta supplied kill lists with the names of thousands of suspected PKI members to the Indonesian military. Declassified diplomatic cables released in 2017 confirmed the U.S. provided economic, technical, and military aid to the Indonesian military while the killings were underway.
D. N. Aidit became General Secretary in January 1951 and grew the party from 3,000-5,000 members in 1950 to 165,000 by 1954 and 1.5 million by 1959. He was captured and summarily executed by the army on the 22nd of November 1965.
The PKI finished fourth in the 1955 legislative election, winning 16 percent of the national vote and nearly two million members. It secured 39 seats out of 257 in the legislature and 80 out of 514 in the Constituent Assembly, with almost 30 percent of votes cast in East Java.