Would it be possible to add a GroupByAuto() method which automatically adds any relevant GroupBy clauses based on the Select(Expression<Func>> contents?

It would, but we won't add it, just as we don't add the missing fields in a groupby using query in the low-level api: grouping on fields just to make the query work isn't how one should write queries: not only is the order important, but it's also possible things like distinct won't work anymore. So if you need this, formulate the query yourself, or write a different query which only has the fields in the projection which make sense.