Now, I already have a tab group named "Test", and I want to quickly add Folder 1 to this "Test" tab group. However, I couldn't find the corresponding setting in the right-click menu of the tab (currently, I can only add it from the tab group editing page).
The right-click menu of the tab only has the option "Save Tabs" (but it can only create a new tab group; selecting an existing tab group will overwrite it).
Or go in the settings to manage the tabgroups (Settings/Preferences/Tabs/Groups) and double click an existing group to edit it : the add tab dialog allows either selection from treeview, recent, favorites and opened tabs
You can also load the tab group into a window, add the tab, then save over the tab group. That's often quicker and easier than doing it via Preferences.