In order to add Additional Crews to your Crews List you will select the "Booked Jobs Calendar" (as shown below).
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Once you can see the calendar, click on the day you'd like to assign crew members to.

Now you'll want to scroll down and click on the small person icon next to the truck that you want to assign the crew member(s) to.

This will then pull up your list of employees on the left. Select the name of the employee and click the "Add" button to assign them to the truck.

Now you should see their name on the right side in the "Assigned" tab. Once you've added all of the employees, click the "Save" button at the bottom and this will save them to the job.