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By trading. They would get into a certain place to meet and trade their stuff.
Also I would improve on kewlgeek's answer: They "bartered" and bartered at posts for supplies and tools. Lastly, ideas got everywhere because, for example, when china ruled vietnam in their ruling times, they exchanged their ideas when they told them to d it that way, like vietnam changed their traditionl way of farming.
They did not have English like we do today to use as a language of business. Often times they would try to point or communicate through gestures and in some communities they created very few, but necessary words that everybody knew.