Complex sentence: The association grew over the years, gained a second office in London, England, and on June 11,2011 the association became an "Institutional Member" of UNESCO through the Canadian Commission for UNESCO which is located in Ottawa, Canada.
Split into simpler sentences: The association grew over the years and gained a second office in London, England. In 2011 the association became an "Institutional Member" of UNESCO through the Canadian Commission for UNESCO which is located in Ottawa, Canada.