Ex-Im Bank is a small federal agency that helps U.S. companies, including small and medium-sized businesses, sell their products overseas, create jobs for American workers, and grow the U.S. economy. The Ex-Im Bank's short-term extension of its charter expired on June 30, 2015.
You can find your member number on the address label of your uschamber.com monthly publication (see example highlighted in yellow below) and in your welcome letter.
Why do you even need a member number? Certain sections of the Chamber Web site are reserved for members only.