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We have got scenario wherein Internal user will create an entry in SharePoint list and will specify external user email ID and then it should automatically add in SharePoint permission group and External user should also get notification or invitation email.
@VikasDadhich - You need to activate external collaboration on your site collection. Then you can use Send HTTP request to SharePoint and call REST API...
Refer details in below thread...
https://emadgabriel.com/2019/01/04/microsoft-flow-add-user-to-sharepoint-group/
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