Make a thourough search of your belongings and room. Try to remember when you last saw it. Be sure to look through all drawers, storage areas and pockets of jackets and pants. Ask your roomates and friends if they may have taken your passport in error.
If these steps do not help to locate the passport, then immediately advise the Group leader who will work with the Tour Leader to obtain a replacement. Always have the photocopy of your passport in a separate location. The photocopy will make the replacement process easier.