By default, MS Graph API has not exposed such endpoints to access SharePoint site groups. But you can consider using below alternative method:
There is a hidden list "User Information List" that stores users (including groups) information. We can get group name through this list.
The endpoint is:
https://graph.microsoft.com/v1.0/sites/{siteid}/lists/User Information List/items?expand=fields
Reference doc:
Best Regards,
Baker Kong
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