After you've created a new group, you'll want to invite people to join your group.
Inviting people to join your group is handled on the Profile page of Tether. From there...
Click on the group you want to invite someone to join
Navigate over to the Members tab within the group
Click the large "Add New Member" button
Either a) copy the group code in order to paste into a text / email, or b) select "Share Invite Code" to be given a number of integrated share options (to text message, email, or other messaging platforms).
You can join a group by pasting the group code into a number of places (see guide here)
Visual references below...
