If you have admin privileges you can add users for any user groups you manage. User groups can have one or more app keys.
Admin status is flagged with this message at the top right of the screen when logged, which can be dismissed at any time:
To add a user, go to My Account>User Group Management. If you manage more than one user group, select the user group you want to edit, and switch from app keys to users:
Then click on add user button, which will bring up this modal:
Note you need to hit return once you have entered the email address for the user. If that email address is already registered on the system, the other details will be populated automatically.
Once you submit the form by clicking 'add user' the system will generate an invitation email containing login credentials for that user. New users should check junk/spam folders in their email client if they don't see the email in their inbox.