Add mail enabled security group to datasource programatically

I sucessfully gave ConnectionCreator rights to such a group on a gateway through the Add-DataGatewayClusterUser cmdlet

Why would you do that? This setting is exclusively for PowerPlatform, not for Power BI. In Power BI you would use Gateways - Add Datasource User - REST API (Power BI Power BI REST APIs) | Microsoft Learn  - with the limitation you already found.

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