We can't find anything definitive but we believe this to be a group of British merchants who traded with Hamburg. The alternative being a group of merchants from Hamburg who traded with London.
At we found a 1645 reference to ".. Isaac Lee of Hamburg, Merchant.” Lee, an Englishman, holding the title of President of the Merchant Adventurers of Hamburg, acted there as a British envoy..." which tends to confirm our understanding.
Wikipedia has a page, which is useful but does not refer to the Merchant Adventurers of Hamburg.
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