In the Freestyle app, go to Users (icon on blue band at the bottom of the screen) > + Invite new user and follow the prompts. If you have the mobile app set up, you cannot set up PINs via the keypad.
You can set up to 20 users (including administrator) in the app.
If you have manually set up the lock using the keypad (and not using the Freestyle app), use this guide to set up the PIN codes.
You can set up to 9 PIN codes using the keypad