I made a mistake in thinking there should be a Roundcube 'register' facility. To do so would be like sniffing up every tree in the forest - every up to date mailserver driver there is. Instead, I will find a register (signup) script for my mailserver (hmailserver) and may or may not provide a link on Roundcube. It is essentially a webserver problem, not a webmail problem.
I have installed the password plugin and am delighted with Roundcube.