What exactly do you mean by grouping and ungrouping objects? If you mean parenting, first select the child, then select the object to which it will be parented. Then press Ctrl + P and select Make Parent.
I would guess that there would be several ways to make a rubber band that stretches. I would probably first model the shape of a rubber band. Insert a keyframe on frame 1 for Size. Advance to frame 50 or so, scale the rubber band in the direction that you want it to stretch, then insert another keyframe for size. It should then stretch for you!
To switch between cameras, first select the camera to which you want to switch, then press Ctrl + 0 (on the keypad). That should switch for you.