Only Account Admins can add users to a project. 

How to add users

1. From the Home dashboard navigate to your Account Settings
2. In the sidebar, click Users. This will open the list of existing users.
3. Click Add User
4. Add users to your Account by typing their email addresses.
   1. You can add multiple by pasting a list of emails into the text field.
5. Email invitations are sent to the users, after you press the Add.
6. If the user should be an Account Admin then check the checkbox in the Account Admin column.