This is not very easily accessible in just the Microsoft Graph. If you had some access to the SharePoint API, you could get the GUID from the "User Information List" - which seems hidden from the Microsoft Graph at this time. That SharePoint API call would be
GET HTTP https://sometenant.sharepoint.com/_api/web/lists?$select=title,id&$filter=Title%20eq%20%27User%20Information%20List%27
Once you have that GUID for that list you could do the Graph call:
https://graph.microsoft.com/beta/sites/{site id}/lists/{list ID from the SharePoint API}/items
That will get you the full list of members, including groups. This is still a hack since the groups you'd have to filter by contentType/name eq 'SharePointGroup' - which seems buggy in Graph Explorer anyways. Trying to programmatically access that, would be difficult at this time.