If a friend, family, or leader has shared a Tether Group Code with you, you can join the associated group by pasting the code into Tether. Here's how...
Navigate to the Profile tab at the bottom of Tether
Click on the "Add New" button to the right of the "My Groups" text
Select Join Group from the pop-up panel
Enter the Group Code in the field provided, and then click Submit at the bottom of the page
Visual references provided below...