In order to restrict job signup to specific people when the criteria aren't easily defined by person type and group, you can use invitation codes. These are user-defined values, e.g. "SpecialCode17", which you associated with jobs and job slots. With VSys Live enabled and job signup enabled within VSys Live, you'll find places in job definitions and in job slot editors to put an invitation code.
Entering an invitation code into a job slot (or a job) makes that job slot or job invisible in VSys Live unless the volunteer enters that code when searching for jobs.
A job slot can have any number of invitation codes associated with it, separated by commas.
When searching for jobs, a volunteer can enter any number (including zero) of invitation codes, separated by commas.
Jobs and job slots matching any of these will be returned if their other search criteria are valid.
If the volunteer enters an invalid invitation code no error is returned (you don't want to give them hints if they're guessing!), and hidden jobs (those requiring invitation codes) won't be shown.