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Looks like you can't use the deselect from the other group, unless you plan to go back and add some back.  The polygroup export just made a decision on this and made some assignments.

I am curious what other process anyone uses for grouping in Blender?

In case anyone later reads this, I found a solution. 

If you are working in Face Select Mode in Blender, instead of using "Deselect", you can "Select", and then use HKEY (or Mesh->Show/Hide Vertices->Hide Selected).  Though it is a Hide vertices command, if you are in Face mode, it will actually hide only the vertices that complete a face. 

For example, group the lShin, and then Select/Hide the group before grouping the lThigh

Woohoo!

(its probably what @RobynsVeil was talking about, but took me this long to get it ;-)

Kind-of, but your solution is quite elegant. Thank you for sharing it with us!

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

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