Invalid object identifier 'user@contoso.com'

What causes it and how to fix it:

• Adding or removing a group member via Graph needs the object ID — a UPN returns 400 "Invalid object identifier"POST /groups/{id}/members/$ref and DELETE /groups/{id}/members/{id}/$ref accept only the user's object ID (GUID); a UPN from a people picker returns 400 Invalid object identifier — resolve it with GET /users/{upn}?$select=id firstCause and fix →

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