Assign existing user from oauth to org

Hi there,
We have a Grafana (5.3.1) with many orgs. We’ve just setup oauth (with auth0) and it is working correctly.

However, I can’t find how I can assign users to org without inviting them through WebUI.
The corresponding call with API is :

curl -sH 'Authorization: Basic xxxxxxxxxxx=='  '' -d '{
}' -H  'Content-Type: application/json'

any hint ?