The planned conference never actually took place, and I don't think Americans being able to attend would have made a difference. Britain, France, and Italy wouldn't allow their own socialist parties to attend. In Russia, the Mensheviks and SR's supported the idea of the conference, but the Bolsheviks disapproved--true, Kamenev wanted to take part but Lenin denounced him furiously.
https://www.marxists.org/archive/lenin/works/1917/aug/29.htm Stalin said, "The road to peace lies not through Stockholm but through the revolutionary struggle of the workers against imperialism."
https://www.marxists.org/reference/archive/stalin/works/1917/08/09.htm and this was the dominant attitude among the Bolsheviks. (Amusingly, from the opposite extreme, Plekhanov also opposed the conference--
he thought the only road to peace was ruthless prosecution of the war. But he was no longer politically relevant.)
So if it had taken place, what would you have? Basically, some Socialists from neutral countries who had little political power in their own countries anyway (that would be even more true of the American Socialists than of those of other neutral powers), the Russian Mensheviks and SR's who were still supporting the faltering Kerensky government, and German Socialists who were either scorned by most foreign Socialists for supporting the German war effort (the SPD) or else the USPD which was still weak and divided between pacifists and revolutionaries.
Incidentally, Lloyd George had originally favored the conference, hoping that pro-war delegates from the UK and from the SFIO in France could use the conference to cement ties with the Russian Soviet (then controlled by SR's and Mensheviks) and persuade it to help keep Russia in the war. Once the failure of the Kerensky offensive dashed hopes that Russia could still play a vital role in the war, he lost interest in the conference and succumbed to popular pressure to deny passports to would-be attendeers.
Of course if the US was still neutral in September 1917, the ability of American Socialists to go to Stockholm would be one of the less significant consequences...