Hi @Matthias Veelaert ,
Thanks for reaching out.
I understand that you are looking to get all users using specific application using Graph API.
You can achieve this by using service principal id in below endpoint.
https://graph.microsoft.com/beta/servicePrincipals/{id}/appRoleAssignedTo
You can get the Service Principal Id from Enterprise Applications by selecting the object id of that application.
Reference to get more details - https://learn.microsoft.com/en-us/graph/api/serviceprincipal-list-approleassignedto?view=graph-rest-1.0&tabs=http
Hope this will help.
Thanks,
Shweta
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