If your friend or family member is next to you, you both open your twinme application, press at the top of the contacts tab. One then “Invite” the other to “Scan” its invitation QR-code, and accept the invitation.
If your friend or family member is remote, tap the “Share” button on the “Invite” tab and pick one of the applications available on your device. When receiving your message, your friend needs to open the included url with their twinme application and accept your invitation.
For more details, see How can I connect with people?