To access your account via Frontpage, they have to have your cpanel username and password. Use a very secure password at least 8 characters, numbers and letts, capitals and lowercase, no full words.

FTP accounts you have created can be limited to whatever folder you tell it to while creating them and its subfolders. BTW, it is against our acceptable use policy to provide space on your account to others. Not only can it get you suspended for doing it, but your friends may cause an issue accidentally with the account which will cause it to be suspended including all domains and email accounts on it.  See http://lunarpages.com/aup.php