You can assign a rental to a user by clicking on the rental in Tenant Turner, then click the Settings tab at the top. The first question on the rental settings page is who to assign the rental to. Here is a screenshot for reference:
If the rental is available for in-person showings, the assigned user should be the person who will be showing the rental most often, as they will be first in line for availability offered. The assigned user will also receive notifications about the rental.