Multiple users can be added to an organization with full admin rights. At the moment this is not a customer facing feature. So, in order to add backend users, you will need to contact support with the email address and your request to link them to your organization. Once we have done that, the person will receive an email with steps to follow to finish setting them up as a backend user.
Once this has become a customer facing feature, we will be sure to notify our customers via our newsletter (and a change to this article).