What year did France invite delegates from the U.S. to discuss a new international version of the metric system?

The metric system, also known as the International System of Units, was invented in France during the French Revolution.  Although it was adopted in 1795, it was not required until 1837.
The Convention of the Metre (also called the Treaty of the Meter), which a delegation from the United States attended, was held in 1875.  It was signed by 17 nations and was designed to provide a common measurement system to aid in science and trade.  
Prior to the adoption of the Bureau of Weights and Measures (BIPM), measuring systems varied not only from country to country, but also from one region to the next.  As the industrial revolution brought more international trade, it was clear that there needed to be a common measurement system.  There are now 48 member nations.  They meet every four years.