1. Go to "My account"
2. Then click "Users" and "+ Add more users"
3. Proceed to fill out the form form with the information and click "Add User". Note that it's not possible to add an email that already has an account.
4. Now the verification email for the user will have been sent, simply click the link in the email, which will take you to this screen to choose your password.
5. User has now been successfully added
Set User Specific Notifications
1. While logged onto the account that you wish to change the settings for, go to "My Account".
2. Choose the notifications this user should receive, in this case I'm setting up a billing users for this account so I only choose to leave billing notifications enabled.
3. Then simply click "save" and the settings will be updated.