@Barry Swales Please find below the answers to your questions:
- Is it possible to set up directory sync using Azure AD Connect without using an on-premise AD?
No, it is not possible to set up directory sync using Azure AD Connect without using an on-premise AD.
- If not, how can the process be automated, whereby a user being added to the B2C AD triggers an invitation being sent to then to join the B2B AD as an external user, as part of a specific group?
You can use graph call to invite B2B users as mentioned in the document here:
https://learn.microsoft.com/en-us/graph/api/invitation-post?view=graph-rest-1.0&tabs=http
Refer to https://learn.microsoft.com/en-us/graph/json-batching for posting multiple invites as a batch job.
- How can the content of this invitation email be customised?
Here is the document that explains how to customize the invitation content:
https://learn.microsoft.com/en-us/graph/api/resources/invitedusermessageinfo?view=graph-rest-1.0
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