Table is just a tool and you should use it when it fits your desired result. If your widgets are supposed to be laid out in a tabular way, then use
Table. Otherwise, it's perfectly okay to position your widgets manually if that's what you need.
Two alternatives to
HorizontalGroup: Just one example of a component made specifically for helping with with your layout. It seems to be what you are looking for.
Two Stage instances: have one
Stage for the game area and one for the interface. Each
Stage will have its own camera, which means you can move the game camera around the world while the interface camera stays still. To control how large each
Stage is and where it is positioned, read about Viewports on the LibGDX wiki.
When positioning manually
One piece advice, though, if you decide to position your widgets manually: don't use absolute pixel coordinates. If you your screen is 400px wide and you want a widget's X coordinate to be 200px, set it to
0.5f * screenWidth, instead. This will allow your widgets to be position correctly, even if the screen size changes and it takes no extra effort on your part.
HorizontalGroup should have two children: a
InterfaceWidget and a
InterfaceWidget will have as children all the things you want to appear in the left of the screen, such as score. The
GameWidget will be the parent of the
PlayerActor and any other actors you have in your game.