Chris Ladoulis , Dec 19, 2003; 05:52 p.m.
You probably won't like the look of the tree with flash, so you'll want to expose for the tree with ambient light. I'd use a long shutter speed with very dim ambient light on your subjects (almost in silhouette from tree lights), then 'pop' the subjects with the flash - with as much distance as possible between the subjects and tree, and with the flash as *close* as possible to the subjects.
You want the light from the flash - which will light the subjects - to "fall-off" as dramatically as possible, so that it doesn't spill onto the tree. Take advantage of the inverse law and get more distance from subjects-to-tree than there is from subjects-to-flash.