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.