After the loss of World War One by Germany and Austria, the Allies convened to discuss in the Paris Peace Talks. It is understandable that Germany and Austria were not allowed to participate in these peace talks. What I don't understand is why Russia wasn't invited to participate. Hadn't they helped the Allies win the war by attacking Germany on the Eastern Front? I understand they were communist, but weren't they a major factor in the victory of World War One?
Because the RSFR (I.e. Bolsheviks) weren't recognised as a legal government. In fact both sides of WW1 had been fighting against them. The Bolsheviks refused to pay the Russian Empire's debts or honour its treaties, so really they had no leg to stand on in terms of being recognized as the successor of the Russian Empire, even if they had wanted to be. Since they believed all Europe was about to erupt into revolution, they probably weren't bothered.