Short answers, pulled from the story.
The principal members were the French Republic, the United Kingdom, the Russian Empire, the United States, the Kingdom of Italy, and the Empire of Japan. Britain, France, Italy, and the United States were also known collectively as the Big Four top powers of the war. The US joined in 1917 as an "associated power" rather than a formal ally, while Russia withdrew that same year after the October Revolution.
The United States entered the war on the 6th of April 1917 primarily because Germany's unrestricted submarine warfare campaign violated American neutral shipping rights. The Zimmermann Telegram, in which Germany proposed helping Mexico regain territory lost to the US, was also a contributing factor. The US joined as an "associated power" rather than a formal ally to avoid what it described as foreign entanglements.
Italy was originally part of the Triple Alliance with Germany and Austria-Hungary but remained neutral in 1914, arguing the Alliance was defensive and Austria's aggression against Serbia released it from its obligations. Under the April 1915 Treaty of London, Italy agreed to join the Entente in exchange for Italian-populated territories held by Austria-Hungary. Prime Minister Antonio Salandra described the calculation as "sacred egoism."
Serbia suffered the highest proportional losses of any combatant, with casualties amounting to 58% of all those mobilised. Including civilians and deaths from disease, over 1.2 million Serbians died, nearly 30% of the country's entire population. Romania had the second highest military fatality rate, losing 33% of its mobilised personnel.
Russia left the Entente after the October Revolution of 1917 and signed the Treaty of Brest-Litovsk with the Central Powers on the 3rd of March 1918. Russia had entered the war on the 1st of August 1914 when Germany declared war on it in response to Russia's mobilisation in support of Serbia.
Japan formally declared war on Germany on the 23rd of August 1914 and on Austria-Hungary on the 25th of August 1914. Japanese forces captured the German Treaty Port of Qingdao on the 7th of November 1914 and occupied German colonies in the Mariana, Caroline, and Marshall Islands. In 1917, a Japanese naval squadron was sent to support the Allies in the Mediterranean Sea.