Sub-accounts require the target user to have created a MineTogether account, you can link the user to the following guide on creating a MineTogether account.
To add a sub-account to your service click on the Sub-accounts link in the sidebar of the panel
Now enter the email address of the user you wish to add (The email address must be the same as the one they used to create a MineTogether account) then click on the + button
You should now see that the user has been added to the Your sub-accounts section where you can configure what they have access to via the Configure Permissions button or remove them by clicking on the Revoke button.
A guide on user permissions can be found at https://www.creeperhost.net/wiki/books/creeperpanel/page/setting-sub-account-permissions