The Germans weren't invited; there's little comparison to be drawn between the French at Vienna (1815) and the Germans at Versailles (1919). All things considered, it was something of a smart choice to leave the Germans out, considering all the arguments and divisions that existed among the allied powers and leaders. Throwing the Germans into the mix would've been like letting the fox into the hen coup, so to speak!

Ok, as other redditors already said, Germany wasn't invited at Versailles, but I recently finished reading a book about WWI and I could tell you what strategy you can use in an alternate history roleplay. The first thing is: try to divide the front. Venice signed separate peace during the war against the League of Cambrai, and survived. If you can, try to discuss with the other ambassadors separately. If you can't, try to show that you think that the U.S. are the "rescuers" of the U.K. and the France. This should bring a feeling of patriotism against the U.S. and, again, breaks the front. Apart from this, I do not know what you can do. Germany was heavily beaten during the war. You can try to blame Austria and suggest to punish her instead of you. But you will have to give away a lot: Alsace-Lorraine for sure, maybe you can try to trade the island of Helgoland (little island but very important and strategic port)for the demilitarization of the Rhineland, but you can't do much. But, of course, try to say "Maybe if you kill our economy an extreme right party will take control of Germany in the next few years..." or something like that. Good luck!

Like /u/LeRoienJaune said try to play up fear of communism. Read into the Spartacist Revolts, which were a communist revolt in Germany that was only crushed by the Frikorps.

Play up the fact that Trotsky is advocating permanent revolution and he might persuade Lenin to agree with him. Play up the fact that giving Poland away might create a weak state that will fall to Bolshevik advances (it nearly did except for the Battle of Warsaw)

Try and advocate to keep the Kaiser along these lines as well, that a stable autocratic Germany is the only thing that might be able to stop Communist encouragement into Central and Western Europe. Also point out that the German Navy is already forming soviets and it looks like a revolution may be imminent. Draw comparisons with the sailors in Petrograd being major supporters of Lenin. Speak to the British separately from the French and tell them a weakened Germany will leave France as the pre-eminent power on the Continent (the British actually tried to tone down French demands as being too extreme.)

Speak to the Americans and appeal to Self-Determination by asking for plebiscites in Alsace-Lorraine and the other territories the British and French want to strip from you. This might divide the allies as i doubt France cares about the wants of the defeated German people, but the USA if they're role-played correctly definitely should.

Give the colonies to the British which will divide the French and Americans against them, the French out of jealousy and the Americans because of Self-Determination.

As for the French- it will be much harder as they were historically much more aggressive in their demands. Cede land in Alsace and agree to a demilitarization of the Rhineland, this might soften their attitude. But overall the objective should be to turn the British and Americans against them, while simultaneously turning them against each other.

tl;dr- Communism and Imperialistic Greed are your best friends here. Use them to full effect.