Hello,
Ok, I'll start from scratch to give you a basic example that pretty much shows the whole process.
I'm assuming you know your way around Blender a little bit so you can switch views and modes as needed.
1. Start with a brand new cube. In object mode, in FRONT view press S then press Z then press 4 then press ENTER. This will scale the cube along the z axis to make it tall. Press TAB to switch to EDIT mode. Press A until all ofthe vertices are selected. Press W and then click on Subdivide-Multi and leave the default value of 2 in there to add more verts to the cube. Press TAB to switch back to object mode.
2. Left click at the bottom center of the cube (at it's feet), press SPACE and select ARMATURE. Press G and then drag the head of the bone up to the center of the box (halfway between the bottom and the top). Left click to submit the positioning.
3. Press E and extrude another bone to the top of the box. Now you have yourself two bones with a joint in between. Now we're at the point where your question might actually get answered!
4. Make sure you are in Object mode. Right Click on the cube(box) to select it. While holding SHIFT, Right click on the Armature to select that in addition to the cube. The selection must take place in that order. After both objects are selected, press CTRL+P and choose Make Parent to Armature. Then select Name Groups and Blender will automatically name the bones as vertex groups.
5. Richt click to select the armature. Press F9 to bring up the edit panel buttons. There should be a series of options under the heading Armature. One the options should be Draw Names. Click on that and you will see the names of your bones that were automatically created when we parented the mesh to the armature. This is helpful for assigning the correct vertices to groups and to the correct bones.
6. Press A to deselect everything, then select the cube. Press TAB to switch to EDIT mode. Ok, we're here at last!
In edit mode, select the vertices of the cube you want to assign to a particular vertex group. Press F9 to open the edit panel buttons. Under Vertex Groups, there should be a spinner button that will list the different bone names that were assigned as vertex groups. Choose the name of the vertex group(bone) that you want these vertices to belong to. Then click on the Assign button. That should do it.
Whenever you want to change the vertices or add new ones to the group, follow step six.
And for the sake of our demo with the cube, enter Pose mode, and rotate the top bone. Should bend quite nicely.
Enjoy your day.
