What you said is true and it will work only if the Word and Excel grouped windows are on top, but if those grouped windows are below 3 other windows then dragging PowerPoint window will not work. In this case, I have to bring the Word and Excel windows to the front and then drag PowerPoint window on top of that group.
What I'm saying is on the PowerPoint window there are 3 dots for Groupy application and there should be an option to add PowerPoint window to the the Word and Excel group. Right now the functionality of Groupy is such that all windows that are part of a group are removed from and do not show up on the list of available windows on the 3 dots of Groupy of other windows.
I think the 3 dots should show all existing (ungrouped) windows on top and then on the bottom (separated via a separator) should show all existing grouped windows, so that I can pick from either list and PowerPoint (as an example) will be added to either an ungrouped window or existing grouped windows.