LP support is correct on the last issue.  If you have created a subweb or subdomain via FP, you can use the FP tools to administer the security. 

Open your subdomain in FP and choose 'Tools' -> 'Server' -> 'Administration Home'.  This will require your login as admin.  You can set up all varieties of user accounts and permissions.  The subweb security can be managed from FP at this point and NOT cpanel.