We don't actually have a notion of "registering" a component.
Usually this refers to implementing WAComponent>>children to return a collection of its children components.
This list has to be specified explictly and not inferred from sends to WAHtmlRenderer>>render: because there are several passes when rendering, each of which needs to know about the subcomponents of each component (through #children), and the pass that renders the body of the HTML (which is where you would be able to "register" them automatically) is not the first.
-Avi Bryant (with some modifications)