To add users, you must have the superuser role. This is usually tied to the service provider role.
You can add or delete users on the user settings page of the superuser account, in the section 'User management'. Click 'Add a user for this account' to add another user.
Note that email addresses of added users must be verified; an email will be sent to the email address you enter, and they will have to click on the link in that email before they can login. If they try to login without having verified the email address, a new verification mail will be sent.