I tried this a few years back. Did not get satisfactory results. Christmas lights are lless than 1W each, i.e. super dim. To get any kind of nice glow/bokeh from the lights, you will pretty much need to eliminate anything on the order of a Speedlight. Either that, or REALLY drag your shutter, like f/16 to knock down the speedlights, and 1 second exposure to get decent light from the Xmas lights. I tried doing this, but we were doing it with a dog that wouldn't hold still, so the best-exposed shot ended up being blurry due to movement.
Maybe set up a few hand-held flashlights or something. I think if I were to do it again, I would take one photo of the tree by itself to get the glow+bokeh, then overlay it with the photo of the family in front of the tree. All from a tripod of course, to keep everything in the same place.
Single portraits like the one gevorg posted are easy enough with the right modifiers to keep the light from bouncing around to the tree. Group portraits are much harder because the area to be lit is so much larger. And if you get your subjects far enough away from the tree to make the lights appear OOF, the tree looks tiny in the distance. Again, not hard to handle with a single-person head-and-shoulders portrait, but when you are trying to do multiple people, head to toe, it's just not really possible unless your tree is huge.
Part of my issue was that I was working with studio strobes (AlienBees B800) which are quite a bit more powerful than speedlites. Even at their lowest setting they could easily overpower the tree lights. I ended up with my best results by adding a couple of ND filters over the strobe to knock down its power, as well as a yellow filter to mimic fire-light, and barndoors to "beam" the light in one direction. I just placed my wife in front of the tree with the single strobe to the side, with her facing into the strobe. Still, that evening was an exercise in frustration. I'll see if I can dig up some of the photos.