Log into your account on the "myicloubonline website". This can be accessed via the login page on the Planet Fitness website by selecting the "View & Update Records" option.
Note: this website has a separate login from the main Planet 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.