By sending someone the 10 character association code. It is in the form: ABCD-EFGH-IJ. This code is provided in the user manual. You can also find it by clicking on "invite someone". FYI the code contained in the manual allows those who use it to become administrators of the framework.
From the website
Click on the "friends" icon and then on "invite a friend" either to retrieve the association code or to send an invitation e-mail.
From the mobile application
When you are on your frame, click on the 3 small dots at the top right of the screen and click on "invite".