I want to pin two cloths together like the beginning of a second rope.002 to the end of a first rope.001. If I use a hook (pin rope.002 vertex group "top-end" to rope.001, it seems incomplete (and doesn't work), like I need to specify what vertex group on rope.001 to hook "top-end" vertex group on rope.002. I'm guessing that I need to do some sort of Hook-Empty to a tail-end vertex group, and then some sort of rigid-body Constraint?
And per Nathan's suggestion I think it went over my head. Here's a screencap and a blend file:
A hook modifier uses additional methods to determine what gets hooked. It only affects vertices that have been assigned to the hook. One way to do this is to use "hook to new object" operation (or any of its siblings, ctrl h in edit mode for me) which will automatically assign all selected vertices to the hook. If you want to just do it manually, you can; enter edit mode on the mesh, select the vertices you want hook, and click the "assign" button on the hook modifier.
Vertex groups work in addition to this. If you tell the hook modifier to use a vertex group, it will only affect verts that are both assigned to the hook and assigned to the vertex group (to the extent that they are weighted to that vertex group.)