:EDIT Came across a webpage I missed before, figured out the grouping thing. But the other question still remanes, how do you make the most even lighting possable?
I don't know how you do it but somehow in radiant you can create what is called a lighting grid which places an ambient light throughout the level. The GTK Radiant user manual tells you how somewhere.
Oh, this only works properly in outside levels, ones with skyboxes. If you have buildings then i think they are all in shadow.