The sub-user is an Agent added by the Admin of the account who can use the system to make/receive calls. Once the agent is added to the account, he/she will get a link to the registered e-mail.
He/she should verify the link and set-up a login password.
Later, the user/Agent can log in to the mobile app or web(dialer.callhippo.com) app using the same credentials and make/receive calls.