If you start with everything in one layer and wish to split them up in groups (Highly recommended) It is an easy process.
Select the layer that holds the elements. You can test this by clicking the view button on and off (The eye symbol next to the layer)
Click on the elements that you want to move. (Make sure the layers are unlocked) If you have other layers and want to simply move the elements to that layer, select Assign to active layer. Make sure you select the layer that the elements are moving to before you do this.
If you want to start a new layer with those elements, select assign to new layer. With this, you can move track or any element to any layer at any time.
One thing to note. You do need to keep in mind what layer you are working in as you add track and elements. If you are on a "main" layer and start laying tack that you want associated with another layer, those pieces will be linked to the "Main" layer, not the layer you expect. (Easy way to see what elements are part of what layer is, the view toggle) If you do this by mistake, simply click those elements, select the layer that you want them to be part of, and click associate with active layer.
You will find that it is very important to lock layers that you are not working with as you get more and more layers added to your design. Locking the layer will keep you aware of where you are laying track and elements and save you from having to move things around later.
Hope that helps. (And makes sense....