Let's go over the steps:
Access your scheduler and hover your mouse over any of your employees. A + icon will appear, click it to create a shift.
Create your shift. Use the search option to search for the Smart Group or Tag that should be assigned to the shift. Click select all. All employees that belong to the Smart Group or tag that you choose will be added to the shift.
Click save and publish to save the shift and publish it. All employees assigned to the shift will receive a push notification. Done!
Note: Smart Groups are part of the Advanced plan and above.