Unzip your jacket as much as is needed as you enter the lift line. If you are trying to keep up with a group that is better than you, you will need to be cautious of overexertion which will lead to overheating and muscle strain. You are the best judge of your own body heat so pay close attention and adjust before you get too hot. If you feel like you are getting cold, it is important to deal with it right away. Put on a hat immediately and try to seal up the area between your jacket and your neck.
Heat rises so block the escape routes at the top.
If your hands are cold switch gloves with someone for a run. If your toes get cold loosen your boots some and allow some more blood to get down in there. The best thing to do to warm up is too start working harder as you ride. If you are tired than go in and call it a day. If it is early get the lead out and start making some short swing turns with aggressive edge sets.