I understand that the Germans, before the Allies (the Americans, Russians, English and possibly the French) signed the documents and then were very curtly instructed to leave the room after which the Allies celebrated. That the Germans would be part of the festivities seems unlikely (although I think I read that even the Germans were offered champagne and caviar) and indeed Keitel would soon be tried and executed. I wonder if in general, in all such ceremonies, the defeated were instructed to leave after signing. A related question is, were only Germans signing the documents? If so, was this because they had lost all European military alliances at this point and of course the Japanese were still holding out thousands of miles away.
Each Axis power signed their own surrender documents. No doubt the protocol for each such meeting was different, with similarities (we're talking military operations here, there's bound to be a guide book on how to arrange such things somewhere in the military manuals of most nations).
What happened to the signing parties would depend heavily on the terms of surrender. The Italians surrendered to the allies for example after they themselves overthrew Mussolini, which would have influenced the attitude of the present allied dignitaries towards them. The Japanese effectively did something similar, the emperor pretty much firing his war cabinet and reducing his status to a far more ceremonial monarch.
I don't think any of them would be happy to take part in a victory party with their (now former) enemies and conquerors. I know I probably wouldn't be (except maybe in case of the Italians, who may well have sent people who had been in opposition to the Mussolini regime).