Currently it is not possible to create additional logins for a single cPanel, however, most sites consist of 3 core elements: databases, e-mails, and files (FTP). You can create logins for your friends to these elements, and they will be able to manage their sites and emails without problems:
1) You can create limited FTP accounts via cPanel -> FTP accounts (they will be able to access only the specific folder that you will assign)
2) You can create e-mail accounts via cPanel -> Email accounts, and then it can be accessed via http://webmail.#master_domain link (without the need to enter the cPanel itself, log in with an email address and email password), also by POP3/IMAP or SMTP.
3) You can create MySQL users via cPanel -> MySQL, and provide the DB name, DB user and DB password details to your friends. They will be able to manage the MySQL database via phpMyAdmin, which can be accessed from http://phpmyadmin.#master_domain (again, without the need to enter the cPanel itself).
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