During the installation of Vive software, you will be asked to sign in to HTC Account. Or, go to HTC account website account.htcvive.com, go to sign in. Follow the instruction there to create an HTC Account or sign in to your social account to connect.
Please aware that to prevent the confusion of login methods, we do restrict user to sign up several accounts with the same email address. Please delete the account that you do not wish to use before you register a new one with the same email address.