Hi @Daniel
To get the external user you need to invite user in your tenant. To get guest users(external) you can use https://graph.microsoft.com/v1.0/users?$filter=userType eq 'Guest' Graph API endpoint. This will give you all external users details in response.
Find Meeting Time and Create Event API will only work for the user within tenant. For external users who does not have license it will not work.
Hope this helps.
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