Currently Shifts don't support guests.
Please refer to this link: Manage the Shifts app for your organization in Microsoft Teams
There have been several posts in Teams Uservoice forum asking for this feature:
Ability to add guests to Shifts in Teams
Allow external users to be invited to Teams Shifts
allow guest users to be scheduled in 'Shifts
Please consider upvoting or leaving a comment if you would like to submit your feedback to Microsoft.
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