'Manage Extra Admins' option missing

Hello

Operating system Ubuntu Linux 14.04.3 Webmin version 1.881 Usermin version 1.741 Virtualmin version 6.03

Recently upgraded to the above versions as I need to utilise the Manage Extra Admins option that we have running on a different instance of Virtualmin, but the option is missing.

Struggling to find much about this option online. An extra module wasn't installed on the second instance to enable this feature, so unsure where/what to look for.

Any help appreciated, apologies in advance if i've missed the 'enable extra admins' box or something as blindingly simple!

Thank you