The command Go TabGroupLoad MyTabGroup TabCloseAll=No adds the tabs in MyTabGroup, without deleting the tabs that are already present. But the command Go TabGroupList is not modified by TabCloseAll=No — the tabs already present are deleted.
How do I select a tabgroup from the tabgroup list and add the tabs to those already present?
If you look in the preferences, under "tab groups", there is a checkbox for that in the settings of each entry ("close all open listers").
If you uncheck the one in question, it should work.