To create a sub-account, begin by creating or locating an existing contacts record. This can be done by going to Control Panel > Contacts/Sub-Accounts Then to convert the contact into a sub-account, and therefore allow the contact to login, just tick the Sub-Account checkbox. Next, enter a password and then tick the various options that you want to grant the sub-account permission to access.
It is important to note that multiple contacts can have the same email address when being used for email notifications only, but if activating them as a Sub-Account the email address must be unique since it will be used for the login.