Hi cala55,
I can totally understand the inconvenience caused. Could you do the tests below with the affected account to help us narrow down the cause of this issue?
1. Configure one Office 365 account of the other four Mac users in Outlook client on the affected user’s Mac to see how it works.
2. Configure the affected user’s Office 365 account in Outlook client on one of the other four users’ Macs to see how it works.
Thanks for your time.
Regards,
Lee Li