Log into your account on the "myicloubonline website".
This link can also be accessed via the footer on the Retro Fitness site.
Note: this website likely has a separate login from the main Retro Fitness website and app.
Once logged in, you can find your membership number on the "My Agreement Info" page.
Note: The membership number is the same as the "Agreement Number" on this page. While you're here, it is a good idea to jot down the address of your primary location. You will need this information to cancel your membership.