You may want to sort groups by the summary, or aggregate, calculation of each group. You can do this using Top/Bottom filters, which are available in v2017.1+.
Top/Bottom filters look for the highest or lowest values in a set, and then put those values in order. You can supply any arbitrary cell in the report in the Top/Bottom filter. So if you set the number of displayed values to "infinity", then no values are excluded, but the ordering still takes effect.
To sort by a summary calculation:
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Ensure that your report has the appropriate aggregate formula in a group footer cell. The cell should return a numeric value in the report output, which you want to sort the groups by.
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From the
Settings menu, click 
Filters. Then click the Top/Bottom tab. -
Check the Limit the report to the top/bottom values check box.
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From the Top/Bottom list, select Bottom to sort in ascending order, or Top to sort in descending order.
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In the # field, enter 2147483647. Why this number? We cannot enter ∞, so instead we want to enter an arbitrarily large number. This is the largest number that can fit without causing a report error.
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From the Value list, select the group footer cell with the aggregate formula.
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If there is a For Each group, click the Delete icon
to remove it.
Keep in mind that this is not a Sort from the sort menu. You cannot use this to make nested groups. This only affects the order in which a group of data is shown in the output. Because this is technically a Filter, this has precedence over the report sorts.