To add a vertex to a drawn lighting group area, go to
Ribbon: DM Electrical->Lighting Groups-> 
Insert Lighting Group Vertex
Pulldown Menu: DM Electrical->Lighting Groups->Insert Lighting Group Vertex
You will be prompted to locate a point either inside or outside of the lighting group area onto which a vertex is to be inserted.
First fence point crossing lighting group boundary:
You will then be prompted to provide a point on the opposite side of the lighting group boundary. The line between the two points should cross the boundary where you want the new point to be inserted.
Second fence point:
A new vertex will be inserted on the boundary. The appearance of the outline will not change, but you will have two new grips that you can use to drag the area into the correct shape.