Depending on how many users you have, you could also create ticket filters. One way would be to create a filter with the rules set to 'Match any' with a list of the users email addresses you want to match as below

And in the filter actions tab have it set to auto-assign the ticket to a specific agent (or team).
Of course, if you have lots of users then this could get a bit cumbersome so won't be ideal, although as you say you've got limited users to use osTicket, it might be an option(?)