To join your rewards program means that they have confirmed their email address and name, created a password, and clicked 'Join Program' for your business specifically.
To turn on this feature, go to your Dashboard then the My Business dropdown at the top right. Choose Settings, and the Advanced tab will be at the lower left-hand side. Scroll down to the bottom half of the page.
You'll want to make sure the 'Track Memberships' feature is turned on to enable access to the other options in Membership Settings.
To prevent customers without an active membership from joining your rewards program, check the box next to:
Only active members can join the rewards program
Hit 'Save' at the bottom of the page to enable this setting to take effect.