To invite a new member, you must have a duo or family membership and be the manager of the main account.
Go to the Members tab in your account
Click on "Invite a member
Enter the email of the person you wish to invite
Click on "Send invitation".
The person will receive an e-mail invitation to join the duo or family. He or she will have to follow the instructions in this e-mail to join your offer.
If you have additional questions, contact customer service directly.