Setting up future meetings with participants using Teams calendar or Outlook integration
Hello
Users configured in Microsoft365 are actually using Azure Active Directory anyway. Microsoft365 doesn't have its own directly, it uses AADC.
If they want to collaborate as a single entity, and certainly use sharepoint they will need to be in a single directory in Microsoft365.
Yes Microsoft 365 Basic is the lowest offering available.
That will allow use of Teams, Sharepoint and OneDrive.
It will create a mailbox for them however you do not need to migrate their domain over to Microsoft365, you can keep that with the existing provider. Microsoft will provide an internal domain (onmicrosoft.com) for use with Microsoft365 services.
Hope this helps!
Feel free to ask back any questions and let me know how it goes.
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