Assuming you are an administrator on your account, you first need to log in to the Office 365 portal and click on the tile labeled Admin. Then click on Active Users on the left side under Users.
Click on Add a user, then fill out the form and click Add
Note that if you do not have any extra Office 365 licenses associated with your account, you will be prompted to buy more.
The user’s password will then be displayed. You may also choose to receive an email with the password. Click Close to exit the new user set up