At the moment, Groups cannot be renamed.
It actually sounds like what you want to use is Layers
Click in Window > Layers. This will open a new Tab (which you can dock anywhere if you like). You can create new empty Layers, add actors into them, rename them, and view the contents of them. Layers are purely for organizational purposes though, and cannot be adjusted in level. They serve as a quick way to select all contained actors at once though.