you can sort by group summary results but you cannot regroup by a group summary as these can alter the summary results.
You could "mimic a grouping" by sorting on the group result and creating a formual field as a group header that uses the summary result in the string.
likely the notion of doing the grouping is a precurser to watning another step after that. Is there more to the question that you assumed would be resolved if you could do the grouping as you wanted?