I want some friends to use my mailserver for their private use, is it possible to give them access to register an account with a password of their choice - without me, knowing it - and without giving them access to the control panel by making them an admin?
Short answer, no it’s not possible. There is no mechanism in place to register an email account. However there are scripts available to do what you ask
are these scripts public, yet? any hint?
Have a look on Github you might find something there
Any links to these scripts?
Simple enough to create email accounts for them with passwords of your choice and then they can change the password using the roundcube interface.