Thank you for posting your question in Microsoft Q&A.
If you have owner role assigned on the subscription that means you can perform any tasks on subscriptions, resource Groups and on resources in Azure.
To perform anything inside Azure AD you need to have Azure AD roles assigned.
Azure role is different from Azure AD roles.
Currently, owner role on subscription is the part of Azure role.
The task you are trying to perform (adding member to group) is within Azure AD. This means you need to have particular role in Azure AD assigned to your user account.
You can go through below article to get more details on permission under each Azure AD role. Depending on the permission you can assign specific roles to your user account to fix your issue.
Let me know if you have any further questions on this.
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