Explode each group in the nested hierarchy except one. You can do this from the Outliner - right click on each group in turn and choose Explode.
Note that if you have already assigned a Group to a layer, when you explode it, its geometry will be assigned that layer.
So when you are down to one level of grouping, with the group open for editing, Select All, and assign all the loose geometry to Layer0, using R-click/Entity info’.
Close the group, and then convert this last remaining Group to a Component (use keyboard shortcut g) to open the Make Component dialogue. Give it a meaningful name immediately. Set the origin to a meaningful point if necessary (it defaults to the bottom left corner in a conventional view, which is often fine). Make sure ‘Replace selection with component’ is checked. Then click Create in the Make Component dialogue to finish making it.
Get into the habit of making a component when you finish drawing each new part of your model, and giving it a name then and there.
And don’t leave any ‘loose’ geometry (anything not grouped or made into a component) in your model except while you are drawing a new piece of the model.
ALWAYS leave Layer0 as the default layer while drawing. ONLY assign a component or group to a layer, never individual edges or faces. But you can assign individual dimensions and text to a layer other than Layer0, even if they aren’t grouped together.