Forum Discussion
unable to change or set admin role for a staff member in Microsoft Bookings
Bookings is turned on by default for customers who have the Microsoft 365 Business Standard, or Microsoft 365 A3 and Microsoft 365 A5 subscriptions. Bookings is also available to customers who have Office 365 Enterprise E3 and Office 365 Enterprise E5, but it is turned off by default. To turn it on, see Get access to the Microsoft 365 Business apps for Enterprise subscriptions.
Maybe the plan you are using for certain members (who are facing this issue) might nor support teams. This can be a reason they are shifted to guest.
Guest | Staff member doesn't have a Microsoft 365 account |
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https://support.office.com/en-us/article/create-your-staff-list-298c529b-407b-4a2b-b2c5-6e77a9d1f07f
Hope that helps.