Employee availability is managed via the Employee Manager. You can find the Employee Manager via "people" in the top navigation bar.
Next, you'll need to click the department that the employee belongs to. You'll find the list of departments on the left side of the screen.
You'll now need to click the employee name in the list and click the "new availability" button located in the availability section.
From there, you can select the days your employee is available, and enter the exact times available during that day. Once completed, save the availability profile using the blue "Submit Availability" button.