Just to let you know, you can only do these things if you are an account administrator or above.
On the Home page, click on the settings icon. From there click on account, this will take you to this.
Click on the people tab, then click on 'add person' and you will then see the below screen.
You will then need to fill out all the fields. You can decide whether the user will be an administrator or a standard user.
N.B Once you save the changes, the user will receive a verification link email which will allow them to access the portal. They will need to create a password in order to access the portal.
Please note you can only use this option if you are an AT&T customer. If you are having trouble creating a user, please submit a ticket here.