edit: I just tried again, for more than a couple of hours. First I had to guess that I had to install MDB2_Driver_mysql. Set up a DNS to match my /var/lib/roundcube and now I'm struggling to understand how I could sign-in since he didn't create any user for me.
As for users, roundcube uses imap and thus do not create new users.
Anyway, GL, and if you want to have some fun, try out PostgreSQL.