When can I remove the El Capitan install drive? I have installed 10.11 from a USB drive, it has run as expected and I am now at the "Create account" screen (input name+username). 

I will now turn the computer over to the person who will do the remaining steps.
Can I remove the USB drive at this point without problems?
Yes, you can now remove the install media and shut down the computer. Once the machine reboots into the Setup Assistant, it's safe to remove the install media.