Every site has a preferred employees list. You can find the option to add an employee into that list by going to a staff member's name in Schedule view and hovering over the little gearwheel shaped icon next to their name.
This will open a pop up window where you can choose a site to add the staff member to. You can then later filter the Schedule view by preferred staff to assign the preferred employees to certain sites.
If you have any additional questions or issues, feel free to email us any time at email@example.com - we'll be happy to help you out!