A Sub account user is a user that can access certain areas of your site. If you have employees that you wish give certain tasks to then you can give them a sub account and grant them access to do certain things on your site.
If you visit the Dashboard > Sub-accounts area and it says "This advanced feature is turned off. Please contact tech support concerning enabling this functionality." then all that you need to do is contact support and let the support person know that you wish to enable sub-accounts and how many users you would like to have. You will create the rest once it is enabled.
When you are ready to add a new user click on the "add account" button.
Then you will need to fill in the user information seen on the next screen.
This is where you will give your new user a username, Password, and grant them permission to whatever area you wish to grant for this new user.
NOTE: when the user logs in to make changes they will need to login with the main username/TheirUserName. For example the user created in the above example would need to login with "wfvideos/test" and the password given to that user.