It is worthwhile checking the official vldpersonals documentation.
Although it is limited, it does provide some useful information like session and member variables that are available for use.
Member VariablesThis tells us that {session.group_id} is the current group id.
You could then use that in a template something like :
<!-- IF {session.group_id} == 13 -->
show the field
<!-- ENDIF -->
You may have to try a few combinations to see if the number, 13, needs to be quoted ("13") or not, and to get it working.
It's good to have a plain template kept unmodified, like webby2, for simple testing such as this, rather than your production site.