For those who do not know, the West entrance to Yellowstone National park is only open to oversnow traffic, i.e. snowcoaches, snowmobiles (guided only), cross country skis, and snow shoes. Depending on your group dynamics and desires, we highly recommend taking a snowcoach or snowmobilling tour in winter. We have consistently heard wonderful things about Yellowstone Alpenguides fro Snowcoach and Rendevous or Two Top for snowmobile tours. Here is a little video shared from National Geographic Traveler on snowmobiling in the park.