You's need the western powers to recognize the Soviet government in Russia (unlikely, since none of them did that until long after the Russian Civil War) and for Russia to wish to join the negotiations (again unlikely, as Lenin was opposed to particpation, calling it a bourgeois congress which was meant to be destroyed by the revolution). So you would need a very different turn of events than OTL.
For example - kill Lenin and the rest of the OTL top ranking bolsheviks. IIRC the tzarist authorities, and later the Provisional Government had a lot of them under arrest for quite a long time. The Bolshevik party thus exists and enters the elections called by the provisional government. There is a great mess-up in the elections, fraud etc. - end result is the Bolsheviks win, but don't get a majority, ending up with them forming a coalition government with the Mensheviks and SR. Kerensky goes out of office, Russia becomes bolshevik through evolution rather than revolution, the civil war is avoided and they are invited to Versailles as one of the victorious powers - the fact that they signed Brest-Litovsk being forgiven.