Are there any instance of US citizens that joined the fighting in the war as volunteers before the USA actually joined the war? I already know volunteers did in fact serve behind the front lines but were there any at the front lines that were actual US volunteers fighting? As in, for example, fighting under the Canadian flag?
If there were would they be added into the existing divisions or would they be put on their own separate divisions to differentiate them from the non-volunteers?
What would (or actually) happened to them when the US joined the war? Did they get sent back to serve in the US army or did they stay as volunteers fighting under a different countries flag?
Sorry if I seem repetitive.
The short answer to your first question is yes, Americans did volunteer to serve with foreign militaries in the years prior to the US formally entering the First World War.
How they served varied a bit. One the most famous examples is the "Lafayette Escadrille", a French fighter squadron composed of American Volunteers led by French Air Service Officers (1). The squadron was active until 1918, so it would seem many of the men chose to remain in French service rather than transfer to the AEF once the US joined the war. American volunteers also served with the British Royal Flying Corps, with a number of them enlisting even after the US formally entered the conflict (2).
A number of Americans (estimated to be as high as 35,000) also crossed the border and enlisted in the Canadian Expeditionary Force (3). The CEF actually tried to raise at least two battalions specifically for American volunteers in 1915/1916 (97th and 237th Battalions, CEF). The 97th was actually called the American legion, but the name was dropped and the battalion eventually absorbed into other units after the American Government objected over fears it could compromise their neutrality (4). See source (5) below for an example of a recruiting poster for the 237th Battalion CEF held by the Library of Congress.
One example of an American serving with the CEF is that of Irwin Hart. Hart was born in Nebraska, and enlisted in the 97th Battalion CEF mentioned above in 1916 in Windsor, Ontario. After the 97th was disbanded, he was eventually posted to the Princess Patricia's Canadian Light Infantry, one of the most famous infantry regiments of the Canadian Army. He was killed in action at the Battle of Vimy Ridge in April 1917. His Service Record has been digitized and is available for viewing on Library and Archives Canada's website: https://central.bac-lac.gc.ca/.item/?op=pdf&app=CEF&id=B4118A-S053
I don't know enough about Americans serving in the British and French Armies to answer that part of your question. Keep in mind that for your average American, the easiest way to get into the fight would've been to do what Private Hart did and cross the border to join the Canadian Military. This saved them from having to make their own way across the Atlantic.
As for what happened to them when the US joined the war, I'm also not sure. I couldn't find much on it when researching for this answer, so hopefully someone more knowledgeable can answer it for you.
(1) https://encyclopedia.1914-1918-online.net/article/lafayette_escadrille
(4) https://www.bac-lac.gc.ca/eng/discover/military-heritage/first-world-war/Documents/infantry%20battalions.pdf (Page 386)
At the outbreak of war in 1914, the US was officially neutral and pursued a more isolationist foreign policy while its next-door neighbour Canada, like other Dominion and imperial allies of Britain, rallied to join the fight for king and empire. Canada would commit 600,000 troops to the front in WWI, out of a population of only eight million at the time.
But, many Americans felt strongly about participating in the war despite their government's position. The Canada-US border was more porous then and tens of thousands of Americans (up to 35,000 by some estimates) crossed the border to enlist in the Canadian Expeditionary Force, the overseas army Canada would send to fight in Europe. While technically not legal, there were plenty of posters along the border informing potential recruits how and where to enlist.
This was initially risky for American recruits, as enlisting in Canadian or other Commonwealth forces to fight in Europe could result in loss of US citizenship. They often hid their citizenship from recruiters, who in turn didn't press them on the issue in order to enlist them. It was an accepted, open secret that Canada wanted the Americans to enlist regardless of the official US policy at the time.
The CEF kept these troops together at first in the 97th Batallion -- even calling them the "American Legion" in 1915. Similar CEF units of Americans were formed across Canada. Germany objected to the presence of these combatants from an officially neutral power on Europe's battlefields, declaring that if America didn't have the courage to declare itself at war, its name shouldn't be invoked. American officials eventually asked the CEF to drop the American Legion label and these troops would be absorbed into other batallions, although they would still be known as such in the field colloquially.
Winston Churchill, then First Lord of the Admiralty, saw advantages in allowing American volunteers to go to Canada or England to enlist:
“Nothing will bring American sympathy along with us so much as American blood shed in the field.”
As part of the CEF, they would have seen action in many of the pivotal battles of the war -- including at Vimy Ridge, often regarded as Canada's "crucible of fire": the WWI battle in which it prevailed where other allies failed and helped to define Canada's emerging national identity.
When the US formally entered the war in 1917, most stayed with their Canadian battalions and had their own government's blessing to do so.
Those who survived the war would retain their US citizenship, but for those who fell in France or Flanders (up to an estimated 3,500), most would be buried there as Canadians along with their brothers-in-arms.