I see that I can add users. That solves my issue.
I notice the UserID field is the same for this user as it is for web2. And both have the same group of course.
So the web2 user created by ISPConfig is not intended to be used for logins, fair enough. Does that mean the web2 account is 'locked' in a similar manner to how Ubuntu creates the 'root' user when you install the distribution?
My VPS system:
(Ubuntu 14.04 LTS, Kernel 3.15.4-x86_64, Apache 2.4.7, MySQL 5.5.38, PHP 5.5.9, ISPConfig 126.96.36.199p5, Webmin, PureFTP & Quota, phpMyAdmin, postfix, dovecot, amavis, clamav, spamassassin, awstats, fail2ban, Jailkit, bind9, vlogger, webalizer)