You can add existing users from associated accounts, or create new users. To create new users, click on the Create New User button on the upper-right-hand side of your screen. You can also skip this step and add users within the account later.
Once you select your users, you can choose the user's user level from the drop-down menu next to the user's name.
In order to save your changes you've made in this step, make sure to go to the Review tab and click save.