What would be needed for Newfoundland to join the colonial side in ARW?
A lot more than what you'd think, and just taking Halifax is not enough. TFSmith121 hit some of the main points, but it also requires conditions to be almost perfect. The reason for that is because thanks to the Treaty of Utrecht, part of Newfoundland's coastline was reserved to the French to allow the maintenance of their fishing industry - the French Shore (which had long been an irritant throughout 19th century Newfoundland history). Also, at that point in time, Newfoundland was also governed differently from the rest of North America, as it was under the direct jurisdiction of the Admiralty as well as economically dependent on the UK through the West Country fish merchants. OTL provides the best example of what Newfoundland's reaction is going to be, even if it sinks the economy. Not only would Britain not give it up; neither would the locals let it, particularly once France gets involved. The loss of Halifax would make Britain depend even more on The Rock than OTL.
In effect, as far as I can tell there's no way Newfoundland could join the rebel side in the ARW. It's too far away, too insignificant in the eyes of the rebels, would be worthless in the long run without the fisheries, and too much trouble. A poignant example - during one of the wars where Newfoundland had the unfortunate distinction of being a battlefield, the French army under M. d'Iberville tried to conquer Newfoundland from the British. However, the residents of the outport settlement of
Carbonear managed to defeat the French, by simply moving the entire settlement across the bay to an island that was largely impenetrable rock and easily defensible. (Some believe that locals learned their craft from a notorious pirate named Peter Easton, who was active 'round these parts.) The French tried to force the outport to surrender, but thanks to all sorts of tricks the locals put on them M. d'Iberville was forced to give up and retreat. Never underestimate the ingenuity of locals to confound all sorts of authority, particularly when that authority is their own.

It would not work to any American advantage, that's for sure.