See this wf : max_min_values.knwf (43.6 KB). Indeed it uses a flow variable, but not in the GroupBy node (it may be possible), but I found a little bit different solution. The ConstantValue column node identifies from what column the min and max values are calculated (using a flow variable: current column name).
gr. Hans
oeps. I missed the discussion in this related forum topic: combining rows values based on other column
sorry