Hey there! In order to add an authorized user to the account, you will need to reach out to T-Mobile through 611, head to the store, use the Message Us link on this page, or T-Force via Facebook or Twitter.
You will need to verify your status as the Primary Account Holder and authenticate with a one time PIN sent to the line of service designated as the primary if you opt for any of the contact methods except visiting the store. If you go to the store, you will only have to show a photo ID.
Manage your T-Mobile account