You can let friends start and stop the server, use the console or manage the files without ever handing over your own account. They get their own login and only the permissions you tick.
- Open your server in the panel and go to the Users tab.
- Click New User and enter the email address of their panel account.
- Tick what they can do. Control covers start, stop, restart and the console. File covers the file manager and SFTP.
- Save. The server now shows up in their panel next time they sign in.
Being a sub-user doesn't give anyone a server of their own, and it doesn't count as one. It's still one server per person and one per project.