You just need and invite from the Group owner, whether you're an existing customer of SMARTY or not. You can only join a Group once you have received an invite from a Group Owner.
Once you have your invite, simply login to your SMARTY account and at the top of your dashboard, you’ll see the option to respond to the invitation. Click through and read the terms and the either accept or reject the invitation.
To become a group member, you must be at least 13 years old or have parent or legal guardian permission