Q: Authenticate external users in azure AD with GraphApi Is there any way to authenticate an external user with Azure AD through GraphApi?
So far I have been able to authenticate users of the active directory with GraphApi but not the external users. I have been able to identify the invited users but I have not been able to authenticate them.
The authentication of users of the active directory is done in Azure AD. Authentication is not done with the AD on-premisse

A: Normally, you can use OAuth 2.0 authorization code flow or OAuth 2.0 resource owner password credential to get access token. 
It works for the users of your tenant and the invited users (work accounts) from external tenants. 
The sample for OAuth 2.0 resource owner password credential

I successful get a token for the invited user (jack@techlearn.top) in my tenant (hanxia.onmicrosoft.com) as following:

And I can use the access token to call the graph api to show all the users in my tenant (hanxia.onmicrosoft.com):

