Original Answer: mostly Americans and Nazis,
Updated: The Americans joined the war on December 8th, 1941 (Pearl Harbour)not gettng to Europe until January 1942. The war commenced 1 September 1939 - 2 September 1945 so American troops joined half way through. They had been providing economic support prior to this though (although this arguably could have been taking advantage of the situation as some consider WWII is what made America the economic pwer it has been in recent times).
Most of Europe (not the Swiss), Asia and Australasia were involved in WWII. African nations participated also - but I am not sure if there were any exclusions. Most of the world participated.
none
All who opposed the Nazis and their allies.
The groups were the allied and the axis. The allied mainly being America,Russia, Britain. The axis being Germany,Italy ,and Japan.
The United States benefited the most from the First World War. France and Germany were particularly negatively affected by the war. The war was fought from 1914 to 1918.
They might make world war z 2 in 2016.And I hope they will.
Jews
There were no terrorist groups in World War I.
In the context of World War 2 resistance groups resisted the Nazis.
British children
bobby joe but
Briton and USA
Communists and Nationalists
gypys
none
African Americans and women
america and germany
British colonist french and irocoise