Make sure you're in the User Panel (click on
User Panel, and then click on the domain with the folder in question if
you have multiple domains). Click on "Password Protected Directories".
If your folder is not listed, click on "Find a Directory to Password
Protect". You should be placed in the domain's public_html folder.
Navigate to the exact folder you want to add a user to by clicking on
the folder names.
Once you find the folder you want to add a user to, click Protect. Make sure the checkbox next to "Use Password Protection" is selected, then enter the username and password of the new user. Pushing Save will add the user.
